Clones, Twins, and Duplicates...
yeah... I've noticed this too... a real rash of programs that do exactly what OS X programs from Apple do. iTunes seems to have it covered, and I've seen a lot of strange little Calendar programs popping up too, but iCal is killer.
I think it shows that some developers didn't have inside info and saw a niche, but once some of these programs got released, the developers were stuck with the developement going on, so just decided to finish and release.
Maybe there's some other functionality I'm overlooking with these programs, but the amazing Apple programs have much of this completely covered it seems to me.