First thing to know, apparently, is that there is a hierarchy among motorcyclists. People who drive the same motorcycle as you (Harleys especially) are more likely to return your hello. People who wear similar gear (helmet or no helmet, leather chaps or denim only, do-rag or braid, etc.) will also be more likely to return your greeting.
It is also reportedly true that some bikers will never return a hello, regardless of your equipment or your gear. In some cases when traffic is heavy or tricky, it might be too difficult for a biker to wave. So bikers are cautioned not to over-interpret another biker's failure to return a wave hello.
That said, here are some of the signals described and explained:
Biker keeps hand on the grip but raises index and middle finger in a peace sign. Safe but also very casual. Like saying, "Hey, dude."
Down Low Peace
Biker drops hand off grip below handlebars and flashes the peace sign. Since the biker took the time to remove his or her hand from the grip to greet you, this is a true hello.
Variant of the Down Low Peace. Biker drops hand off grip below handlebars and gives an open-palm. The return of the greeting is sometimes held as the bikes pass so that it appears as though the bikers would give each other low-fives but for the extra necessary distance between them.
Sport Bike Point
Biker raises hand without moving the arm and points at fellow biker with a flick of the wrist.
The Geeky Hi!
Biker lifts hand from grip and raises hand into the air and waves. This increases wind resistance and could result in injury. It is also how newbies or nerds -- or I who have never driven a motorcycle -- would wave. Serious motorcyclists say that drivers of mopeds and motor scooters greet each other this way. In other words, if you want to be cool -- and don't want to slow down -- don't wave like this.
Biker removes hand from grip and lays it flat on the thigh. No peace sign, no open palm, just a big obvious nothing. Supposedly, this comes from motorcycle gangs and some riders will still interpret this as an open sign of disrespect. Newer riders might instead see this as a sign of tiredness and that a driver is merely resting his or her arm.
Biker points one finger down to the ground point to the problem to warn of debris on the pavement ahead.
Palm of hand taps top of helmet several times in succession alerts other bikers to a police officer ahead.
Middle finger proudly extended. No guide to hand signals would be complete without this one.
Motorcyclist who takes more risks than necessary. A show-off and, in the end, probably not that good of a rider. Some think that riders who hardly wear any safety gear and only skimpy things like flip-flops and a flappy sleeveless T-shirt are squids by virtue of their dress alone. There's even a way to signal another biker that a squid is on the way: drop hand with palm down, and wave the fingers in a tentacle-like fashion. Sport bike riders wearing helmets so they don't get pulled over, but eschewing shirts altogether and leaving their legs and ankles bare. Some refer to riders like this as squids; others refer to them as "future body bag occupants;" still others simply wince at the thought of skin grafts.