Notify me when it's up!
I love these simple one-purpose sites. There's no clutter, no learning curve, and they just do what they say. You might argue that Notify Me When It's Up should be combined with Down For Everyone, but where will it all end? A comprehensive desktop app with stats displays, accounts and iPhone support? Ok, probably not. But in a complicated world, it's nice to find someone who just wants to notify you when it's up.