meebo is a multi-IM client (in beta version right now) that is Web 2.0 enabled: a really cool use of Ajax!
I tried it and seems to be functional and really usable (it's in beta, don't forget). Probably a very powerfull alternative for user without a Laptop or a Network-enabled palm.
The most interesting features? it use 1024 bit encryption RSA for Ajax communications!
Source, Consequentia Mirabilis.