I've been a long-time user of Google Talk. No-nonsense, lightweight (installer was sub-2MB!) chat client that talked to just about anything else. And unlike open-source Jabber clients at the time it had a killer feature: centralized and searchable message archive when XMPP crowd was only starting to think about XEP-0313.

Fortunately XMPP has caught up in the past decade feature-wise (except cross-platform video calls I guess) and I managed to convince some friends to use Conversations but this could have played out differently had it not been for G+.