Answer homework questions after school, hold virtual office hours, or just stay connected with your students outside of class!  It helps to be using the same messaging application as them. 

Here are links to popular instant messaging programs your students may be using.  Some have more robust features than others, but if you just want basic text-based chat they all function similarly.  When in doubt, try it out! 

If you already have accounts with any of these sites, you may want to create a separate screen name to give out to students.  Remember to keep it professional!

AOL Instant Messenger, aka AIM

Yahoo! Messenger

MSN Web Messenger

Windows Live Messenger

Google Talk, aka Gmail Chat

Skype

ICQ (sounds like "I seek you," cute, huh?) - more popular in Europe than the US, but you will still find American students using it

Meebo - a handy program that lets you use AIM, Yahoo!, MSN, and Google Talk through a single interface. Less clutter!

Having trouble deciphering your IM conversations?  Check out this Cyber Slang Guide