CoSOS (cousin of SOS) is an open source extension for WinDbg focusing on .NET memory fragmentation (!gcview) and threading issues (!wfo, !tn).
Typically, the extension is not available side by side to any other DLL, so it is usually loaded with .load x:\full\path\to\cosos.dll. It requires that SOS is loaded  and currently works with 32 bit applications only.