Google Talk

Google Talk is Google's XMPP-based IM service, which has (obviously) Gmail and Google/Login integration. It can only be termed VoIP charitably, since it doesn't use standard protocols.

Good Points:

• Clean UI
• Gmail address book integration
• Third-party clients supported from the outset (list is incomplete. since others work too).

Bad Points:

• Windows-only (duuuuuuuh!)
• No SIP (less firewall hassles, I suppose), no mention of ENUM anywhere, etc.
• Doesn't work well across my VPN setup (and likely neither through most Microsoft shops' default firewall setups)

Fun Points:

• The Wumpus Game Easter Egg

• MGTalk - J2ME Jabber client with extra Google features.
• Developer Info
• Melo's initial post, follow-up - lots of detail (he's the expert on XMPP, not me).