What hand signals are there for communicating with others when cycling in a group?
Yeah, what Stephen says is about all I know (though I don't run with the racing crowd).  Pointing at road debris is commonly done, but probably useless, since it doesn't allow time to react -- I prefer to just yell "Trash!", especially now that I have a bad arm and the signaling is more difficult.

Using the hand signal (left arm (US) extended with elbow bent downward at about a 45 degree angle and with open palm facing backwards) for slowing is a bit more useful than pointing at road debris, but shouting "Slowing!" or "Stopping!" is still a better signal -- easier to accomplish, and more likely to get the attention of the rider behind.

And, of course, there's the shouted "Car back!" signal given from the rear (no possible hand signal here) when a vehicle is overtaking a group of bikes, and "On your left/right!" shouted when overtaking another bike.

(I'm sure that some cycling groups have additional agreed-upon signals, but I don't know of any others that would be generally recognized in the US.)

For motorists, I use the basic hand signals -- left arm extended for left turn, right arm extended for right turn.  I occasionally use the "slow" signal (described above) when needed to signal that a motorist needs to use caution (because, eg, of cyclists ahead that he may not be able to see), and I've used it once or twice when occupying a full lane to signal that I'm going to retain occupancy (such as when negotiating a single-lane bypass through a construction site).  I also sometimes use the left (US) hand extended, palm down, with a repeated downward motion to signal "slow down", when there are cyclists or some other hazard ahead.

When following others at all closely, cyclists need to learn to watch the feet, rear derailer, and rear brake caliper of the cyclist ahead.  These signal speed changes, and careful observation will clue in the cyclist behind even when the cyclist ahead fails to somehow explicitly signal.