On the guitar end, something to spark-up the clean and give feedback and chugging some oopph! on the distortion... or bass just something to even out the strings a bit and tame errant over-zealous fingers. I'll have to look into that one. Thanks
Diamond Compressor is really nice for a smoother sound and somehow just makes your tone better. You aren't going to get that tight compressed slap sound if you slap though because it doesn't do enough compression. If you slap a lot then it's not right for you.
The most versatile compressor I've used is the Forest Green Compressor, it's got a lot of sounds in it and they just put out a PCB version that saves you a bunch of money.
Go to Compressor Reviews for a bunch of compressor reviews. I think if you find something good for bass you will be able to make it work on guitar IMO.
The Demeter Compulator is an awesome compressor! That being said, it's not a compressor made for a squashed sound. It's very transparent sounding. I use mine to tame some of the volume spikes that I get with some of my effects.