A Garbage Collector for C and C++
http://www.hpl.hp.com/personal/Hans_Boehm/gc/
GPS: I've been impressed by how well it works. I think that it might be interesting to build a Tcl-prototype that use the Boehm-GC for Tcl_Obj management. It could possibly eliminate thousands of lines of code and improve the quality of Tcl/Tk.
There was an excellent article in C/C++ Users Journal about the Boehm-GC.