Open Watcom could use someone with C++ knowledge to help finish there version of the C++ standards library.
Its about 50% done sadly the project lacks developers so it wont be finished any time soon unless someone chimes in
I have helped squash as many bugs as my limited understanding allowed me to, but i only recently started dabbling in C++ so i cannot help them in that regard.
If anyone here can im sure they would appreciate the help though

.
As for benefits Open watcom is a multiplatform compiler that allows making programs for several operating systems on one platform (you can make Linux executables on say Windows and vice versa).
OS support includes Linux / QNX / OS/2 / dos / Windows 9x - NT / AXP NT atm.
The C compiler is mostly complete and most of the bugs i helped squash was in the w32api.
The C++ compiler is working but because the STL still needs a lot of Work not everything builds with it yet, i did manage to compile most of wxwidgets with it though so its atleast in a usable state.
If you want to help Development you can make commits to
Productivity is a state of mind.