ICQ
ICQ (I Seek You) is the first Instant Messaging program dating back to 1996. It was invented by five Israeli Students in Tel Aviv and AOL currently owns ICQ. They purchased it from Mirabilis for $407 Million.
What I wrote about ICQ in the past is not applicable. AOL has virtually stopped promoting it, and prefers to promote it's own client, but ICQ lives on for nostalgia reasons. 10 years later, and my friends and I still remember our ICQ numbers, but we don't use the program anymore.
When you sign up, you as assigned your UIN (Unique Identification Number). The higher the number, the newer you are. You can buy low numbers over eBay.
Until 2003ish, ICQ was the best Messenger and Chat client with the best features, and it docked nicely to the side of your desktop. Always there and reliable. You could even replace the person's username with their real name to help you keep track of your contacts.
New versions started to be released that took up more computer memory and also included more and more advertising. Competition came along, and people made the jump to new programs.