I've led many group rides with the members of this forum, and there are a number of hand/leg signals we use regularly:
Left arm out (usually pointing) - left turn
Right arm out (usually pointing) - right turn
Left leg out and back in quickly - moving to left lane/left side of lane
Right leg out and back in quickly - moving to right lane/right side of lane
Left leg held out - obstacle in road on left side
Right leg held out - obstacle in road on right side
Left arm out with palm facing rearward - stopping
Left arm moving up and down with palm out and facing down - slow down
Tapping helmet - cops in the area
Thumbs up - everything's OK (or if you're the last rider in the group, everyone's accounted for)
Turn signals and brake lights are used in place of or in conjunction with hand signals for turning/stopping, too. If I'm leading, I'll often use hand signals and turn signals, but if I'm just following, I'll usually just use turn signals to indicate turning. Also, on occasion, I will use my arm and finger to point at an obstacle in the road rather than use my leg, but I usually use my leg for that purpose.
There may be other signals we use occasionally, but those are the ones I can think of off-hand. We ride staggered, but don't really have a hand signal to correct people who aren't doing it correctly. Maybe we should add that one to our arsenal, as we seem to need it often.
Oh, and if I see someone with a blinker on by accident, I'll use the blinker hand signal, which I've learned as: make an "OK" signal (last three fingers up, with index finger and thumb making a circle), then open and close the gap between your index finger and your thumb like you're pinching something. This doesn't appear to be a universal signal, though, so many people don't know what I'm trying to tell them.

I can't find the reference I learned this one from, but I saw it somewhere in a list of hand signals.
So yeah, I guess we do use a fair number of signals. Got to communicate somehow if you want to get 20 riders to do the same thing safely.