I had covered this topic before but it is time to update a bit since I just found out that AOL also have the web IM (Instant Massenger) as well. Therefore, start from the beginning:
In case you are in a situation where the computer you are using do not or cannot or do not wanted to install certain IM software but you wants to chat with your friend on MSN, Yahoo, or AOL here is your solution.
MSN Web Messenger, this one is, of cause, for MSN users. However, true to Microsoft's style, it will only works when you are using Windows OS and preferably with Internet Explorer as your browser (on the website it said it will also works with Netscape 7.1 but I doubt anybody still using that one)
Yahoo Web Messenger, the name saids it all. This one seems to be tougher to use than the other... It seems to utilize different port than usual, thus, it probably will not work in case of office environment where they often blocks most other internet activities other than web browsing.
AIM Express, for AOL users. This one seems to be the most versatile among the three. It works in closed environment, just like MSN Web Messenger (my praise to that) and it will works on most web browsers as well.
I hope this post is useful to whoever read this :)