Proteus is a multi-platform chat client, just like Adium. It used to be shareware, but now it's FREEWARE. When you launch it though, you'll be given the options to enter a registration key, use the free trial or purchase the product. Select purchase and go through the eSellerate process, as if you'd be buying. Don't worry, it won't ask you for credit card details. It's just a process that's been built into the software, which the developers promise to remove in the next release. Enjoy!