We are using TFS and VS 2010.
I have been working on a project that is using TFS as source control. I have quite a few dlls that I have downloaded (such as log4net) and
I've tried various methods for dealing with this and have settled on dropping required dll's in the bin folder and making sure they are included in the project for source control. I've heard people say this might not be a good idea but nobody has provided good reasoning for it and it's worked well for me.
My second choice would be to carve out some space on a network share and organize the various 3rd party dll's there. You can put your files in folders with verion numbers to keep things straight and everyone should have access to everything they need, so long as everyone uses the normal network paths as a reference.
Adding a seperate folder within the project is also workable but seems messy, since you end up with extra files that you don't want included in your release.