T00ls.IM is the original IM service based on XMPP and one of the key nodes on the XMPP network.
To request an account, please follow the instructions here.
To log in, use an IM client like Adium, ChatSecure, Conversations, Gajim, Jitsi, Messages, Pidgin, Psi, or Swift (a full list is here).
For help, see the FAQ.