Here’s a brief list of what I’ve found comparing some of the most popular multi-protocol IM clients.  I’ll try to update this here and there when I have time!

Windows

Linux

iPhone

Mac OS

Web Interface

Web Widget

One login for all accounts

Tabbed Chat Windows

not for

iPhone app

Video

Tokbox

Some

work needed

Only

for gTalk

VOIP

With

Plugin

Tokbox

Some

work needed

(pay only for

calls to landlines)

Works Even

with iPhone app!

Only

for gTalk

Merge Contacts

Facebook

LinkedIn

MySpace

AIM / Yahoo! / MSN / XMPP

Supports, No Support, Web client may be used, Must pay for feature