Overnet is simple to use and is pretty reliable. It doesn't like Zone Alarm much though and doesn't like Nortons Ad stopper. It is supported by AD-Ware but running Spy-Bot s&d after installing it rids you of this and has no effect on the program. The program itself still shows ad's in the top corner but is unobtrusive.
Gnucleus, is freeware with no spy-ware attached it has a smaller following; however, it is growing by leaps and bounds and the source code is available to anyone who wants it. C++ language and gnutella back bone.