Tomorrow is Yahoo!'s first open Hack Day. They've previously had company wide hack days, where employees spend a day hacking away on projects they want to work on, typically using APIs or other internal libraries to create a project outside the typical work tasks.
I'm looking forward to the event very much. Even if I do have the feeling it may become a subtle Yahoo! recruiting effort. I mean, what better way to find talent than by inviting hackers over and watching them work?