Gmail users might be interested to have a look around Settings -> Labs to find add-ons the Gmail team (and also 3rd party developers) have made. Some of them could potentially be fairly useful, though one or two are slightly unusual... For example, "Mail Goggles" will pose some maths questions before a late night email is sent just to make sure that it's not something you'll regret the next day... Hmmm.
Now Gmail just have to sort out filtering the really obvious junk mail that creeps into my inbox everyday. They used to be so good. Come on!