Free Alternatives is a great database of free software. In addition to tremendous depth in its listings, Free Alternatives also includes rankings and a search features that enables you to quickly find free alternatives using the commercial product name. For example, search for "Word" under "Proprietary application" and you will find Open Office, KWord, and two other options. Searches for other expensive programs such as Photoshop, Dreamweaver, Cakewalk, and Cool Edit Pro yield an impressive number of free options. Be sure to check out the education section.
Also, for those of you looking to get started with open source software, check out the OpenCD.