The Linux Equivalent Project lists Linux alternatives to dozens of popular Windows applications, from Adobe Photoshop to ZoneAlarm.
The idea here is to show potential Linux adopters that making the switch doesn't necessarily mean saying goodbye to their favorite software. You can use Scribus in place of PageMaker, Quanta Plus in place of FrontPage and so on. Needless to say, the site includes links to all the "equivalents" so you can check 'em out directly--and encourages you to submit your own.
The Linux Equivalent Project
Find Linux equivalents to Windows software