(if you have any suggestions for additional links let me know)
ICQ - As in I Seek You...my favorite way to know when friends/family are online...far more functional than AOL's Instant Messenger will ever be!
Buddyphone - A great way to talk one on one with others using your computer's microphone. (works best in combo with ICQ)
Excite Voice Chat - If you sign up here (it's free) they do a voice chat with the capability to have up to 10 people in a "room" talking - seems pretty cool.
AOL Instant Messenger- Personally I don't care for it (it can only do one at a time messages, nothing more), but some of the family members use it