Google's IM and Voice Chat service was launched today. Their service uses open standards for instant messaging. That means I can pretty much use any client with XMPP/Jabber support to connect. The voice calls are signaled using a proprietary protocol as of now but they do plan to document it and to support SIP later. More about that in the Google Chat Developer's FAQ.
I use gaim for IM and I was able to connect to Google chat with it. Gaim supports multiple protocols like Yahoo, MSN, ICQ and AOL so I don't have to run multiple clients. While Google has released their own client for Windows, they do provide links to other clients like gaim that can be used with their network. Although gaim supports Yahoo and MSN, they had to reverse engineer these protocols. Once in a while these networks change their protocol and the gaim developers have to crack the changes and release updates to let users connect again. While Yahoo and MSN try hard to lock users to their networks and officially released clients, Google allows any client and is also making efforts to let you make voice calls to users on other providers (Earthlink and Gizmo to start with). Trust Google to shake things up when they venture into a new field.