Android/iOS: You could previously use Foursquare in a non-annoying fashion, but it was still more of a tool for tracking your personal wanderings and maybe finding your friends. With the 3.0 upgrade, now out in the Apple and Android markets, a new "Explore" tab makes stumbling across neat places much easier.
In some ways, the "Explore" tab is simply an organised, optimised, friend-finding version of the "What's Nearby" feature in previous Foursquare clients, but that's exactly what was missing: a way to see what's nearby, with reviews and friend recommendations taken into account. The Foursquare blog goes into (much) deeper detail on what went into Explore, but use it a few times and it becomes an interesting way to look at what's nearby, in your own town or when travelling.
It all started with a check-in: the vision for #4sq3 and beyond [Foursquare Blog]