I like LJ Talk. It’s an instant messaging service which uses Jabber, an open protocol also known as XMPP, meaning you’re not locked in to particular software like you are with MSN. Yes, I know that a lot of software has reverse-engineered the MSN, ICQ etc. protocols, but they still break when the owners change the rules. I use bitlbee to talk to people on LJ Talk; others use Pidgin, Kopete, Psi and lots of other software.
It’s also tied in to your LiveJournal – your LJ friends are automatically added to your IM list. However, as an example of this lovely open protocol, LJ Talk isn’t all that great. It doesn’t integrate into the LJ website like Facebook Chat does (though the inability to turn FB chat off irritates the crap out of me), it’s not well publicised like Google Talk is, and it seems to have gone back to being a walled garden where you can’t add non-LiveJournal contacts. If you want a Jabber account to play with, I suggest jabber.org.
Recently 
greyeyedeve has been having real trouble with LJ Talk. It seems that the encryption certificate used for LJ Talk has the wrong hostname (www.livejournal.com rather than livejournal.com) and hence some IM clients such as Kopete will refuse to connect. Most Jabber clients will let you specify a server, and connecting to www.livejournal.com (though keeping the Jabber ID as username@livejournal.com) will fix the problem.
