We've been recommending IMO.im for chatting with Olark on mobile devices (iPhone, Android, Blackberry). It's nice because they don't require a separate account, you can just log in directly to Olark via XMPP/Jabber.
However, it can be difficult to see what pages people are currently on (takes a few clicks). There are others out there:
IM+
Ebuddy
BeeJive
Anyone have favorites? Any tips for going mobile?