Lone Wolf has good aftermarket Glock stuff.
Glock makes a 3.5 pound trigger assembly that's pretty good. There's also a little mechanism that makes the trigger reset a bit faster. A friend of mine has a G19 with both modifications installed, and seems to love it.
I have a stainless steel guide rod, with a bit beefier spring in one of my Glocks. That's the only modification that I've done. It's helped reduce the felt recoil from the gun, and I haven't had any reliability problems with it.