What is the best IM client out there for both mac and pc?

http://www.trillian.im/ trillian is a popular cross platform IM client with versions for PC, Mac, iPhone, Android and Blackberry Smatphones

Most work on many platforms (I believe Adium is a client that can access many networks on Mac, Pidgin on Windows and Linux), and in the worst-case scenario you can always use meebo.com (a website that lets you IM through many different networks without having to install anything).

uhh, skype obviously

It doesn’t really matter so much if they have Mac or PC, but rather Yahoo vs AIM vs GChat vs MSN, etc. But Pidgin lets everyone talk to everyone – ie I can talk to someone who only has Yahoo chat while I only have GChat by using Pidgin. Skype would be my other recommendation. I would not recommend Trillian, because it puts tons of crapware on your computer.

We used ICQ for years, is that still around???

I am with @Aethelflaed; Pidgin is good, Trillian is bloatware with a side order of crap.

@rooeytoo Yes, and Pidgin can do it.