They all suck for one reason or another, but these are the ones I've tried & my opinion of them.

WebMessenger: no tabbed chat, subscription based, can't send/receive files (photos)

IMOV: Ugly interface, no tabbed chat, can't send/receive files

IM+: ugly, no tabbed chat, can't send/receive files, initial investment is expensive

Agile Messenger: horrible tech support, expensive & subscription based, still has a few bugs

Live Messenger: Flaky connection problems, only supports MSN

None offer the ability to keep you logged in if you lose your wireless connection & deliver your messages to you when you come back online as Verichat did.