1. 10 Minute Mail - Instead of giving your email to every site out there that asks for it, or making up some random email only to find out that a confirmation email will be sent, use 10 Minute Mail. 10 Minute Mail generates a random email address that expires in 10 minutes (duh) the time can be extended to whatever you need, and you can receive emails through it for the time it exists, making it perfect for the unsavory signup at a site that's sure to spam you till you die.
2. Songza - Songza lets you search for just about any song you want to hear (I haven't found one it couldn't find yet), create temporary playlists, rate and share songs to your heart's content.
3. Gmail - While this is kinda a given, it is worth repeating for the sake of those that are just coming out from under a rock.
4. Google Reader - RSS readers are everywhere, I know, but combine a RSS reader and Google and you have the best RSS reader you'll ever see.
5. TinyURL - Instead of sending a link to a friend that takes five rows to paste, send a shortened link that they could remember (if they decided not to paste it right away).