Miranda IM is an all-in-one standalone app that allows you to merge contact lists and communicate with people on AOL Instant Messenger, Yahoo! Messenger, ICQ, and several other messengers out of the box. Module add-ons can make Miranda work with Skype and add other features. (Note that the Skype add-on in particular requires that Skype be installed on your PC--though not necessarily running--as Miranda interfaces with it.) Miranda is open-source and free of charge.
Unfortunately, Miranda's user interface leaves something to be desired. When adding a new instant messaging client, there's no instant way to know if you put in the correct username/password. The icon representing your specific instant-messaging program is dark when you're not logged in. After some trial and error, I determined that setting the status of a particular IM account from "offline" to "online" triggered a login attempt. This doesn't seem intuitive: Other apps, such as Yoono, have a separate login screen during account setup, and this login must be valid for the app to set itself up correctly the first time.
Miranda IM displays everyone from all your instant messengers together in online alphabetical order, then offline alphabetical. It would be nice if, instead of mashing together contact lists by default (likely creating duplicate listings), there were an option to view contacts from specific IMs in different tabs, or merge contacts so that only one representative of each name is displayed.
Frequent users of many different IM programs will be bugged by the omissions in Miranda's all-in-one interface. If you get instant message spam, there's no way to block or report the spammer like you can in the individual clients: the best you can do is delete his or her name from your contact list, if it gets there. And, of course, this is text chat only: none of the IM video capabilities in the individual clients carry over to Miranda.
For those who use only surface capabilities of instant messengers, Miranda IM is for you; however, if you'd like more granular control, Yoono Desktop supports most of the same instant messengers, is also free, and is much more full-featured.