Until not so long ago the only things I 'checked into' on a regular basis were airports and hotels. That's started to change recently with my discovery of location-aware mobile social apps Foursquare and Gowalla which enable me to 'check in' to locations pin-pointed by my phone's GPS, share that location with my friends and earn points/rewards along the way.
It's not a new idea (Dodgeball was experimenting in this area way back in 2000, albeit using SMS) but it's an idea which seems to have suddenly come of age, thanks primarily to advances in the smartphone market, with GPS now coming as standard on most handsets and mobile apps enabling interaction with web services without the need to open a browser.
The other magic ingredient in this new breed of mobile social app is the leveraging of game mechanics to incentivise uses to keep using the app and checking in to new locations. Foursquare awards you points for every check-in (which are then stacked up against your friends' totals on a leaderboard), badges based on the patterns of your check-ins and mayorship for a location's most frequent visitor (with real-life freebies at some venues). Gowalla uses stamps in a virtual passport as its central conceit, with pins instead of badges and items which you can virtually geocache.
Aside from the pursuit of kudos, both apps not only let you know when your mates check-in somewhere (which really comes to life on the iPhone now apps can use push notifications) but also function as a handy guide of neat stuff to do nearby. Whilst this triple-whammy of functionality makes the apps hard to describe in a nutshell (Foursquare describes itself as "50% friend-finder, 30% social cityguide, 20% nightlife game"), it potentially gives them the edge over apps which tick only one or two of those boxes (e.g. Google Latitude, Brightkite, Dopplr).
One of the biggest challenges for apps like this is coverage; most of the negative comments about Foursquare in the app store are about the limited number of cities it covers (currently Bristol, Brighton, Edinburgh, London, Manchester in the UK), although the ability to check in anywhere is apparently on the way (functionality already provided by Gowalla).
In terms of which app currently has the edge, Gowalla probably squeaks it thanks to its gorgeous UI and iconography although it's a close call and it's unlikely to remain a two-horse race for long as other developers wake up to the potential size of the prize in this area.