Trillian is a very handy tool if you want to send instant messages to people who use different clients without having to install and switch between all the various clients.
It lets you connect to ICQ, AOL Instant Messenger, MSN Messenger, Yahoo! Messenger and IRC in a single, sleek and slim interface (that's what they say on their homepage and it's true!).
Trillian can also send 128 bit encrypted AIM and ICQ messages and lets you communicate more securely than with the original clients that way.
One problem is that (rightly or wrongly) some of the IM services (esp. AIM) don't like Trillian hooking into them and so every now and again throw the Trillian users out so you will have to update Trillian to work around this.