I'm an instant messaging junkie. Mostly I use the IM in Skype but I can also be seen on the AOL IM platform through iChat. I know lots of folks that also use GoogleTalk, Yahoo's IM offering, and so forth. This means, in general, these folks will have many different IM clients open.
Adium (sound like a new cholesterol drug) allows you to access many services in a single client rather than having multiple windows open.