After using “Trillian” for quite some time, I was looking for a better multi-protocol instant messenger - one that supports more accounts (like e.g. Google Talk) on an open source basis being highly configurable. I think “Pidgin” works really nice.

• Official “Pidgin” website:
  www.pidgin.im

Note:
You should also have a look at the “Guifications” plugin.

[Update from 2008-03-31]
I’ve always wanted to change the behaviour of the “return” key - it should insert a blank line rather than sending the message.

Here is one way to do it (on Windows XP):

A. Create (or find) the file %APPDATA%\.purple\gtkrc-2.0
B. Insert the following code:

# Make Pidgin send message with CTRL-RETURN
binding "my-bindings"
{
    bind "Return" { "insert-at-cursor" ("\n") }
    bind "<ctrl><ctrl>Return" { "message_send" () }
}
widget "*pidgin_conv_entry" binding "my-bindings"</ctrl>

This was not so easy to find on the Pidgin website.
Unfortunately, “<alt>s” (like Skype) wouldn’t work.