I just found: osalt.com.
It has mappings between open source and commercial software so you can easily check to see if there is an open source alternative to an app you use.
They seem to keep their alternative list paired down nicely. This is a great advantage as there can be a ton of alternatives to some of the more popular commercial applications.
The problem is lots of them are dead projects that didn't get very far or are so "beta" to not really be useful at all. With luck, they will just maintain a list of the top alternatives.