Is there a Visual Studio addon, or a method present already, that allows me to recursivly compare a project/solution/folder with what's in source control?
Note: Source control product agnostic (a.k.a VSS)
The reason i assume is there is a source control agnostic solution is that Visual Studio supports Microsoft's MSSCCI API. So Visual Studio can perform the operation no matter what source control provider you have.
Well, the Visual SourceSafe client allows you to do this, at least the one with Visual Sourcesafe 6.0 did. It's interactive (you didn't say if you wanted batch functionality).
If you are source control product agnostic, then any of Subversion (and its great Windows shell addin TortoiseSVN) , Mercurial, and Bazaar will allow you to do a diff of all or part of a complete repository.
VisualSVN is a Visual Studio addon for Subversion.
I don't know of any SCM product agnostic VS addon, and the chances of finding it are pretty slim. There are too many differences between SCM products for a useful generic addon to be built.
I'm not sure that it can register as an addon (haven't tried) but I'm running WinMerge as my comparer inside SourceSafe, though it only runs on a single file when used from that perspective. It can do folder comparison as well which I've recently used with great success due to a major version change on our product plus maintainance on the old version.