Windows Live Messenger 8 BETA
was invite only, but now its been released and anyone can download it from
here.
It looks nice, although it does have a few bugs (such as the fact that it often crashes on sign-in when Internet Explorer 7 (also beta) is installed. Otherwise a nice upgrade to the popular MSN messenger program.
I like Google Talk as well because its much smaller (takes up about 6Mb RAM compared to 25+MB ram for MSN) and simpler. It is more stable, its just a shame that not as many people use it.
Windows Live Mail BETA is still difficult to get onto. If you go to the site linked above you will see where to sign up for it, but they say it takes at least 2 weeks and there's no guarantee that you will get on.