Throughout Lifehacker's archive of over 14,000 posts, we've mentioned hundreds of free web and desktop apps that help you get things done—but which are the best? As we wrap up the year 2007, it's the perfect time to put together an authoritative guide to our favorite pieces of free software and web services for common computing tasks on every platform. After the jump, check out our favorite freebies to date in over 20 categories and counting, as we add your suggestions to the list.

Application Launchers

• Launchy (Windows, open source)
  Previous coverage: Download of the Day: Launchy, Take Launchy Beyond Application Launching (Feature), Tweaking Launchy (Screenshot Tour)
• Quicksilver (Mac OS X, open source)
  Previous coverage: A Beginner's Guide to Quicksilver, Advanced Quicksilver Guide, The Quicksilver Video Extravaganza, Nine Time-saving Quicksilver Triggers, Top 10 Quicksilver Plugins, Quicksilver Creator on the Future of QS (Exclusive Lifehacker Interview).

Backup Utilities

• SyncBack (Windows, local backup, freeware)
  Previous coverage: Automatically Back Up Your Hard Drive with SyncBack (Feature)
• Time Machine (Mac OS X, local backup, built into Leopard)
  Previous coverage: The Simplicity of Time Machine Compels You, Schedule Your Time Machine Backups with TimeMachineScheduler
• Mozy (Windows/Mac, online backup, up to 2GB free)
  Previous coverage: Back Up Files with Mozy

BitTorrent

• uTorrent (Windows, Free)
  Previous coverage: Organize Your BitTorrent Downloads with uTorrent
• Transmission (Mac OS X, open source)
  Previous coverage: Download of the Day: Transmission
• ted (Windows/Mac/Linux, free)
  Previous coverage: Automatically Download Your Favorite TV Shows with ted
• See also: A Beginner's Guide to BitTorrent, Intermediate Guide to BitTorrent

Bookmarks

• Foxmarks (All platforms with Firefox, free)
  Previous coverage: Synchronize Firefox bookmarks with Foxmarks, Yes, Foxmarks beats Google Sync when it comes to bookmarks, Back up and Sync your Firefox Bookmarks to Your Own FTP Server with Foxmarks
• del.icio.us (Web, free)
  Previous coverage: LIfehacker posts tagged del.icio.us

Calendar

• Google Calendar (Web)
  Previous coverage: Black Belt Scheduling with Google Calendar, Better GCal Firefox Extension (Exclusive Lifehacker Download)

Desktop Search

• Google Desktop (Windows/Mac OS X/Linux, free)
  Previous coverage: Master Google Desktop search, Get More from Google Desktop

Disk Space Visualizers

• WinDirStat (Windows, open source)
  Previous coverage: Visualize Your Hard Drive Usage
• JDiskReport (Windows/Mac OS X/Linux, freeware)
  Previous coverage: Visualize Your Hard Drive Usage

DVD Rippers

• DVD Shrink (Windows, freeware)
  Previous coverage: One Click DVD Rips with DVD Shrink and AutoHotKey (Feature), Copy DVDs to Your iPod (Feature)
• Handbrake (Mac OS X/Windows/Linux, open source)
  Previous coverage: Back up DVDs on a Mac

File Syncing

• FolderShare (Windows/Mac OS X, free)
  Previous coverage: Download of the Day: FolderShare, Sync Your Firefox Extensions and Profiles Across Computers with FolderShare
• SyncToy (Windows, free)
  Previous coverage: Synchronize Folders with SyncToy 2.0

Image Editing

• Paint.NET (Windows, free)
  Previous coverage: Download of the Day: Paint.NET 3, Power replacements for built-in Windows utilities
• Picnik (Web)
  Previous coverage: Edit your images online with Picnik, Edit Your Photos Directly in Flickr with Picnik

Image Viewers/Managers

• IrfanView (Windows, free)
  Previous coverage: Download of the Day: IrfanView (Windows)
• Picasa (Windows/Linux, free)
  Previous coverage: Organize Your Digital Photos with Picasa (Feature), Publish and Map Your Photos with Picasa (Feature)

Instant Messenger

• Pidgin (Windows/Linux, open source)
  Previous coverage: Download of the Day: Pidgin Instant-Messaging Client, Encrypt Your Instant Messages with Pidgin (then Gaim)
• Adium (Mac OS X, free)
  Previous Coverage: Download of the Day: Adium X
• Meebo (Web)
  Previous coverage: Connect to popular IM services with Meebo, Meebo Adds Video and Voice Chat to Web-Based IM, Transfer Files over IM with Meebo

Macro Makers

• AutoHotKey (Windows, open source)
  Previous coverage: Turn Any Action into a Keyboard Shortcut

Media Players

• VLC (Windows/Mac OS X/Linux, open source)
  Previous coverage: Download of the Day: VLC Free Media Player, Rip DVDs with VLC
• MediaMonkey (Windows, freeware with paid upgrade)
  Previous coverage: Download of the Day: MediaMonkey (Windows), Whip Your MP3 Library into Shape, Part I - Level the volume, Whip Your MP3 Library into Shape, Part II - Album art, Alpha Geek: Whip your MP3 library into shape, Part III: Metadata
• foobar2000 (Windows, freeware)
  Previous coverage: Download of the Day: foobar2000, Roll Your Own Killer Audio Player with foobar2000, The Beautiful and Varied World of foobar2000 (Screenshot Tour)

Password Managers/Helpers

• KeePass/KeePassX (Windows/Mac OS X/Linux, open source)
  Previous coverage: Securely Track Your Passwords, How to import saved Firefox passwords into KeePass, Download of the Day: KeePassX, Make KeePassX Leopard Compatible
• Firefox's built-in password manager (Windows/Mac OS X/Linux, open source)
  Previous coverage: Secure your saved passwords in Firefox (Feature)
• BugMeNot (Web)
  Previous coverage: Bypass Web Site Registration with BugMeNot

PDF Readers/Writers/Editors

• Foxit Reader (Windows, free)
  Previous coverage: Download of the Day: Foxit Reader 2.0
• Preview (built into Mac OS X Leopard)
  Previous coverage: Do More with Preview in Leopard
• CutePDF PDF writer (Windows, free)
  Previous coverage: Create a PDF from any document
• See also: Top 10 PDF Tricks

Personal Finance/Money Managers

• Wesabe (Web)
  Previous coverage: Organize Your Money in 2008 with Wesabe
• Mint (Web)
  Previous coverage: Is Mint Ready for Your Money?

Start Pages

• NetVibes (Web)
  Previous coverage: Trick out your Netvibes
• iGoogle (Web)
  Previous coverage: Show Us Your iGoogle (Screenshot Tour)
• See also: The Start Page Showdown

Telephony Managers/Helpers

• Skype (Windows/Mac OS X, free)
  Previous coverage: Lifehacker posts tagged Skype
• GrandCentral Virtual PBX (Web)
  Previous coverage: One Phone Number to Rule Them All, Consolidate Your Phones with GrandCentral (Screenshot Tour)

Text Editors

• Notepad++ (Windows, open source)
  Previous coverage: Top 10 Windows Downloads, 6. Notepad++ (text editor)
• TextWrangler (Mac OS X, freeware)
  Previous coverage: TextWrangler Free, Full-featured Text Editor for Mac

To-do List Managers

• Remember the Milk (Web)
  Previous coverage: Get Organized with Remember the Milk
• Rainlendar (Windows, free with paid upgrade)
  Previous coverage: Rainlendar desktop calendar and todo list
• Anxiety (Mac OS X Leopard, free)
  Previous coverage: Anxiety Task Manager Integrates with iCal and Mail

Virus Killers and Malware Cleaners

• AVG Free Edition (Windows, free with paid upgrade)
  Previous coverage: AVG Free Anti-Virus
• ClamWin (Windows, open source)
  Previous coverage: Free Windows virus protection with ClamWin
• Ad-Aware (Windows, free with paid upgrade)
  Previous coverage: Cleanse thy PC with Ad-Aware
• See also: How to fix Mom and Dad's computer

Web Clippings

• Google Notebook (Web/Firefox, free)
  Previous coverage: Clip and collect the web with Google Notebook, Getting Things Done with Google Notebook
• Scrapbook (All platforms with Firefox, free)
  Previous coverage: Save and annotate the Web with Scrapbook

Wikis

• PBwiki (Web, free)
  Previous coverage: PBwiki
• MediaWiki (All platforms, open source)
  Previous coverage: Set Up Your Personal Wikipedia

Zip File Extractors

• 7-Zip (Windows, open source)
  Previous coverage: Lifehacker Top 10: Free Windows Downloads
• The Unarchiver (Mac OS X, free)
  Previous coverage: Download of the Day: The Unarchiver

Phew! Surely we missed one or two or 16 items on this list, so now we pass the torch to you, dear reader. What should be here that isn't? Let us know in the comments, and we'll add to it as we go.