I highly recommend Sneakemail.com and have been using them for over a decade. They let you set up keywords, like "foo", and then you can mail anything-foo@sneakemail.com (or snkmail.com, since a few sites block the former domain) and it creates an alias. On the Sneakemail web UI you can enable/disable aliases and see recent mail. An alias can forward up to 5 people or so, and when someone sends you mail, if you reply you'll reply through Sneakemail.

Of course it's also worth mentioning Mailinator.com (one-off throwaway aliases, but public) and 10minutemail.com (super-disposable temporary email addresses).