More favorite tools: Gowalla
URL: http://gowalla.com/
So, full disclosure, I found out about Gowalla because they signed up to use Urban Airship. In other words, Gowalla uses Urban Airship, and I'm a cofounder at UA.
HOWEVER
That does not dilute Gowalla's awesomeness.
Gowalla is kind of like FourSquare - you check in at locations and your friends are notified that you have done so. I view these apps as an easy way to invite my friends to my location. And, yes, there is the game aspect of it. With FourSquare, you're competing on points and attempting to get mayorships. With Gowalla, you get items - beautifully done images, really - that you can swap with items at the spot you're at or others.
FourSquare has several advantages over Gowalla, such as a larger user base, restrictions to city limits (though I do enjoy seeing British friends checking in, it's not exactly useful information) and an API. However, it seems that Gowalla is working hard on these things, so I'm looking forward to them as well. Gowalla also requires that you be within a certain distance around the spot you're checking in at, which is nice.
Much like a luxury car vs. an economy car, or a Mac vs. a PC, it's the little features that are so well done that appeal to me about Gowalla. They have a great design and the app is rock solid. I'm definitely a fan.