Google have released a messenger program. I tried it myself just now, although i have no one to talk to yet. First impressions are not bad at all. It's based on the jabber system which means aim and triallian gaim and that messenger that comes with mac are compatable with it.
It's clean and advert free. Perfect for anyone who just wants to talk ( not chat )or exchange info as it has no emoticons (yet) and no file transfer. NO silly msn plus sounds or anything. However it has a mic telephony system like skype. Bad news for skype.
You sing in with your gmail account, you can add people who dont have one and it will invite them to gmail.
It does not log anything atm or encrypt chats at this stage