These are the programs I would use if I didn’t have a Mac. I invariably end up installing many of them whenever I have to work on a PC. Many are open source and all of them are free:

• GAIM — If I couldn’t use iChat, I would use GAIM. It works with all the major chat networks but without all the spammy ads.
• PDFCreator — PDFCreator allows PC users to print to PDF, as you natively can on a Mac.
• OpenOffice — Not perfect, but in my opinion a great competitor to Microsoft Office and you can’t beat its price.
• Crimson — A great free text editor with tabs and syntax highlighting.
• FileZilla — A free FTP/SFTP client.
• Firefox — An awesome Internet browser that arguably runs better on a PC than it does on a Mac.
• 7-Zip — A free utility for zipping and unzipping almost any file type.
• iTunes — Even if you don’t have a Mac, don’t suffer with Windows Media Player. Download the Windows version of Apple’s iTunes.
• VLC — This cross-platform media player plays almost any movie.
• Paint.net — A free, Windows-only alternative to Photoshop.
• PuTTY — A free telnet/SSH client.