I have a love/hate relationship with AIM. On one hand, the tech folks are great and they've done quite a bit to make things easier for folks who might want to implement their protocols in software and, on the other hand, the marketing folks still think AIM is a great Trojan horse to convince people to buy dial-up.
Now, thankfully Open AIM is offering the best of both worlds: free access to the API in Linux, OS X, etc. and a refreshing lack of nasty marketing. With Open AIM, you can build location-based plugins as well as VOIP solutions and all kinds of silly stuff all for the price of a download.