Slick is a good IM app for Symbian S60v3 devices
It’s free and runs great on my N75, uses little data and is only 721K. It supports ICQ, Jabber, AIM, MSN, Yahoo and GTalk. I can confirm that the ICQ, Jabber, and GTalk protocols work. The ICQ protocol is difficult to test yourself, because it only lets you be logged in one place, so you can’t send a message from your desktop client to your phone, but it retrieved my contacts fine and showed who was online properly, so I assume that works too. Another nice thing about it is that it uses a small font, so the threaded message view is actually useful.
Slick is simple, fast, and functional. Eqo take note.
If you’d like VOIP functionality also, you can try Gizmo5, but I haven’t used it and the other two I tried, Eqo and Fring, were failures. Fring might be fine if all you’re doing is VOIP and using Slick for IM, but will probably run your battery down even faster.