alternativeTo is sort of the Internet’s version of a thesaurus. It attempts to show apps that are similar to the app you search on. As such, this is a very handy tool for any knowledge worker.
For example, I recently posted a story about Springpad to my blog. alternativeTo has an entry for Springpad. It correctly lists Evernote, Microsoft Office's OneNote and even a personal favorite - DEVONthink; but no Diigo. So. It is not perfect, but still useful.