Things to look for:

speed of chat:
delays are atrocious

space:
how much you can write

saveability:
save your chat content

voicemails etc: 
Excite offers free voicemail and fax. 

webcam and voice enablement, broadcasting and conferencing facilities

free SMS text messenger

Free Instant Messengers

Free email is nice.

Free private chat sessions are nicer, but there are drawbacks.

Just download the software to your machine. Both parties (to the conversation) must have the same downloaded software installed, and must be online at the same time for to engage in online conversation.

AOL Instant Messenger

MSN Instant Messenger

Yahoo Instant Messenger

Excite Instant Messenger