There are several apps out there that will broadcast your location to your friends but there are many times you don't everyone on your friends list know where you're at and these apps use a lot of battery power since they're constantly updating. Spacetime gives you the same functionality without the drawbacks.
Instead of casting a wide always-updating net Spacetime waits until you have a specific need to find someone. Open the app, select a person in your contacts (or enter a number) and that person will receive a SMS saying you're requesting their location and to click on the included link. Assuming they do so and have an internet-capable phone you'll shortly see their location on your Maps app.
The app is iOS only but it doesn't matter what kind of smartphone your contacts have. The location only works once, though, so if you want to find that contact again or they change locations you'll have to send another SMS. This is both to preserve your contact's security and to save battery life.
Spacetime is a free download from the iTunes App Store.