Note: Some of these may get out of date, but I've centralized my tools list at jasonn.com/tools

This list of free essential Windows programs will make the life of any developer or power user much easier.

• 7-Zip handles practically any zip version you could imagine. It will decompress tar, gz, rar, zip, bz2, rpm, arj, cab, cpio, 001, and deb files. It also allows for plugins to add further file type support.
• Adobe Acrobat Reader or Foxit PDF Reader are essentials due to the ubiquitous nature of PDF documents. Foxit is lighter, much faster, and a smaller install. It's preferable on a system with limited resources, like a laptop. Adobe's tools can be resource hounds.
• Avast is completely free, lightweight, and effective. No antivirus is impervious to bad (user) behavior. However, this tool allows you to add an element of protection while not requiring you to open up Active X controls or slow your PC down with a bunch of bloatware.  I especially like the script blocking tool, which will stop a multitude of website based assaults on your Windows PC.
• Audiograbber, because we all listen to mp3s and it makes it very easy to rip them from your CDs.
• Burnatonce allows you to burn ISOs, and has a very simple interface to help you create CD images. It's not a robust point and click interface like so many commercial CD burning tools. But, seldom fails to perform. It's a nice, tiny CD burning tool.
• Filezilla, from the people that brought you Mozilla (and Firefox) gives you point and click command of FTP and FTP over SSL and SSH2.
• Firefox is the smaller, Mozilla family browser that allows tabbed browsing and various plugins and extentions that makes browsing much more pleasant and useful.
• Irfanview is a very utilitarian image editing tool that allows you to quickly create a batch of thumbnails, modify images, crop, modify size, change color scheme, file type, etc.
• Notepad ++ is an extremely lightweight and useful text editor. It offers tabbed file edits and color coding for code tags.
• Open Expert can be useful for Windows power users because it "enables you to specify any number of suitable applications for each file type."
• Open Office handles most any Microsoft Office document with more efficient use of resources and most importantly, FREE! It includes a spreadsheet, presentation, document (writer), html WYSIWYG, and other document management essentials.
• Putty is a free Telnet, SSH1, and SSH2 client.
• Tight VNC makes sharing desktops (relatively securely) between Linux, Unix, or Windows PC and laptops painless. It takes all of five minutes to set up a server and client. The windows ap is extremely simple and very light on resources.
• Trillian is a simple, though somewhat heavy, instant messenger that allows you to combine all your IM worlds in one tool: MSN, Yahoo!, AOL, Google, and Jabber (used by Google's IM network). It doesn't have painful popups, which is my primary reason for using it over any proprietary IM client.
• True Crypt Open gives you the ability to hide data and encrypt it. If you are carrying any type of work related data with you, it's essential you protect it. Encryption isn't bullet proof, but it adds a layer of security.
• VLC video player or BSplayer that plays MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, and a ton of other codecs.
• Winmerge allows you to do a "diff" or compare to similar files to see quickly what the differences are and select your preferred version of each difference, saving your munged version to a new (or old) file. There's much better programs out there, but this one is extremely simple, mostly functional, and free.
• XviD allows you to view Divx. If you're going to watch these video files, it's important not to install some adware, etc.
• Songbird is an Open Source music player.  The nice thing is that it supports open standard codecs and will likely grow in popularity and functionality due to its Mozilla roots.  From the website: "Songbird supports MP3, FLAC, and Vorbis on all platforms; WMA and WMA DRM on Windows; and AAC and Fairplay on Windows and Mac."

Feel free to contact me if you have other suggestions for this list.