- QIP - the original version was a pure ICQ client with tabbed chatting, multiple instances, cool features (merging history, per-contact alerts). The new Infium, though, is multi-protocol with even more features. QIP is light - very light - and has everything out of the box. Supports plugins. Infium natively offers VoIP. Oh, it's Made in Russia, as well ;)
- Miranda - is an Open-Source, multi-protocol, plugin-based instant messaging program. It has an immense feature-set, but after installation all goodies need to be manually added. One of the drawbacks is that Miranda is clumsy in multi-user mode. It originally asks for no password at start and one should hold Ctrl upon launch in order to bring up the user account manager (guess there's a plugin for that, but I have never used one). It is rather meagre in UI design, though.
- Trillian - the most beautiful client. It is multi-protocol, feature-rich and is so sleek in style. But it has a great problem with windows - default chat window size is very small; it can be changed manually for every contact, but upon tabbed-chat style it goes back to normal. I wait for the new Trillian Astra and who knows - it might be the new favourite kid on the block.
So, guys, I urge you to try one of these. Miranda and QIP (not Infium) have installation-less versions - just extract in any folder and have a look!
You will see no advertisements, no slow performance. What is more - any of these (QIP in lesser extend) are meant to be all-in-one IM clients.