I use Pidgin for IM. I really like that it has an IBM/Lotus SameTime integration plugin called “MeanWhile”.
It’s not perfect. Pidgin’s MeanWhile doesn’t handle logged in users properly.
To fix that on Ubuntu, go here, and download the package for your install – 32-bit or 64-bit. Then execute
sudo dpkg -i libmeanwhile1-1.0.2-3*
… and restart Pidgin. You may need to hold or lock the libmeanwhile package from an update.
There’s an outstanding issue about file transfers that still requires a solution.