Don't forget kqueue/libdispatch. Yes, with epoll and friends you can essentially reproduce kqueue's functionality, but there are some really interesting things you can do when all that functionality is available in one place and you have a library like libdispatch written on top of it.
Right, libdispatch is a OSXism which I've never used because I like to write portable code. -- but if you know that your code is always going to run on systems which support it, it's definitely something which can be useful.
