There are many instant messaging applications available for s60 smartphones. Some are java and others are native apps. Some support one service alone e.g. iCQ or Yahoo. Your best bet is an application that provides unified acceess to several services.
Here’s my list, in no certain order:
1. eBuddy (java)
2. Palringo (native)
3. Nimbuzz (native)
4. Fring (native)
My favourite is Fring. The user interface is smooth, and it unifies 12 services, including: Yahoo, Googletalk, ICQ, Skype, Twitter, Gmail notifier, Facebook, among others.
The Gmail notifier is particularly useful.
While Nimbuzz is almost at par with Fring, it lacks the Gmail notifier and I just haven’t been able to get used to the user interface.
But, hey, take your pick. Now you can stay in touch anytime, anywhere that your network has coverage.