There are many alternatives:
- Gaim: for Gnome Desktop (supports a lot of protocols)
- Kopete: for KDE Desktop (ídem)
- AMSN: written in Tcl/Tk (only for MSN protocol)
- KMerlin: form KDE Desktop
- Kadu, GNU Gadu, EKG: messangers compatible with Gadu-Gadu protocol
- Psi: Jabber client using Qt
- Tkabber: is a feature rich Jabber client written in Tcl/Tk
Gaim is easiest to use, and AMSN is most similar to MSN Messenger(c).