Here is a list of useful free software that I use :
Windows only
7-Zip – a free alternative to WinZIP, and way of avoiding dreadfully slow zipping in Vista
xTerminator – to delete mail from a POP server without having to download to your inbox
Clipomatic – records your clipboard history so you can paste old items
Expresso – a regular expression development tool
FastFontPreview – quick way to look at fonts
GanttProject – free tool for gant charts (java)
HeidiSQL – does some things you can’t do with MySQL Query Browser
PDF Printer – makes windows a bit more like a Mac so you can print directly to PDFs
ISOrecorder – burn DVDs from ISO disk images
PureText – paste text into any application without the formatting
Toad for MySQL – powerful MySQL GUI with auto-completion and tabs
Large Text File Viewer – view really big text files (like 1GB)
Google Cloud Connect for Microsoft Office – lets you work on a document in Office, but syncs it to Google Docs so you can collaborate
Switcher – provides a feature similar to Expose on the Mac – to view all open windows
Nomad.NET – a tabbed alternative to Windows Explorer
WinMerge – compare files or folders for differences
Windows and Mac
jEdit – advanced editor, I’ve only scratched the surface of it but one of the useful features is you can hold down CTRL and select a square block of text (it ignores the flow of lines on the page so you can leave out e.g. the first 3 characters from every row)
VirtualBox – virtualization, ideal for running testing environments
CloneVDI – a tool to clone disk images used by VirtualBox
kSar – java app to graph sar data (sar can monitor performance on linux)
NetBeans – one of my favourite PHP IDE’s, now faster than Eclipse
Klok – Personal time tracking, really easy to use with nice interface
Mac only
TextWrangler – a powerful general purpose text editor