Can I use Yahoo messenger with AOL messenger?

You could create an AOL account as well and then use a multi-protocol client like Pidgin to sign up to both of them at the same time.

Usually each service cannot communicate with another. Gtalk, MSN, ICQ, YIM (yahoo), and AIM, when used traditionally (installing the IM software from the service provider directly), cannot communicate with each other. It is only through the use of multi-protocol clients that communication between different IM services can be done.

One web based one that I like is Meebo. You can create a Meebo account, then link up all your IM accounts to it. Then, when you log in to Meebo, you’ll be logged in to all your IM accounts and can talk with all your friends regardless of what IM they are using. One problem with this is that you may not have access to all the IM features (like playing a certain game together, or screen sharing), and Meebo is web-based only, not a downloadable program. But other than that, it’s really great. I often use it to get around firewalls that block MSN directly.

Yahoo and MSN can talk to each other.

Pidgin +1