Skype chat plugin for Pidgin
The plugin comes in Linux and Windows varieties. Linux users can install the plugin using a DEB package or by copying a file to the Pidgin plugin directory. Windows users just need to copy a DLL to the plugin directory. There's also a Skype plugin for Adium for Mac users.
Now the bad news. First, there's no support for voice calls, which should come as no real surprise since Pidgin doesn't support voice chat for any protocol yet. Second, you need to run Skype in the background or you won't be able to login to your Skype account using Pidgin. Which kind of defeats the purpose if you ask us.