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