Alternative To is a great site that lists alternatives to software. Don't want to use Microsoft Office, look for an alternative to it (Google Docs, Zoho, OpenOffice, etc).
This is very useful in finding free, or inexpensive alternatives to paid applications. It's also a good way to find other similar software to find features that you need.
You can search all or by platform - Windows, Mac, Linux, Web-based and by category.
I posted about this last summer, but I felt it was good enough to repeat.