I have had a couple of folks point out Visual Studio Team System 2008 Team Foundation Server is no longer on MSDN Downloads. Turns out it "is" still on MSDN Downloads - we have just moved it to where nobody would think to look and just to make it more challenging removed it from the MSDN search....Sorry about that!!!
Until i can get this fixed the easiest way to get to Team Foundation Server Workgroup or Trial Edition is: Downloads > Developer Tools > Visual Studio > Visual Studio 2008 > Visual Studio Team System 2008 Team Foundation Server Workgroup Edition.
Or even easier simply click on this link: Visual Studio 2008 and select the version of Team Foundation Server you want.
I am hoping to get this back into search and under the Team System category ASAP!!!!