Invite Share - get and share invites to websites running those invite-only private betas. It's a great way of discovering bleeding edge web apps. The neat bit is that the more invites you share, the higher priority you're given when requesting invites.
Xinu - quick stats about any website - age, Alexa rank, incoming links, Technorati authority... etc. UPDATE: got taken down due to getting Techcrunched but another version is up at Xinu Returns.
Heuristics for User Interface Design - Oldie but goodie: Jacob Nielsen's ten principles to keep in mind when designing interfaces.
uncluttr - A clean, usable interface to Amazon.com written using their web service API. Very cool!
co.mments - centrally track responses to your comments left on other people's blogs. Useful service, shame about the url...
Tailgate - the future of online advertising? I'm not convinced people would be comfortable entering their credit card details into a banner ad.