Sending IMs to a specific user?

I'm trying to use llInstantMessage, but I don't have the key of the specific user I want to send it to. Is there a way to look up avatar UUIDs? Or do I have to place an object owned by the specific avatar in the sim and use a complex system of GetOwnerKey to acheive this?

There are external services like the one at w-hat (http://w-hat.com/name2key) which you can call with llHTTPRequest, but I wouldn't recommend them if you want to guarantee delivery of the message because they only contain a subset of the avatars in SL. In most cases you'd be best to collect the key yourself using a sensor or a touch event (or whatever makes sense in your situation).

Yeah, um...noob question...how do I get the key with a sensor?

EDIT: Never mind, I think Name2Key works. I just don't know how to call it with llHTTPRequest...

http://rpgstats.com/wiki/index.php?title=LlHTTPRequest name2key example near the bottom
http://www.moopf.com/blog/?p=16 example using moopf's database

Getting key with a sensor:

string NAME = "Masakazu Kojima"; // whose key you want
default {
  state_entry() { llSensor(NAME, NULL_KEY, AGENT, 96, PI); }
  no_sensor() { llOwnerSay( NAME + " not found nearby." ); }
  sensor(integer num) {
    llOwnerSay( NAME + "'s key is " + (string)llDetectedKey(0) );
  }
}