I've been working on an iOS app that tries to make learning vocabulary a little more fun (ie. feel less repetitive). At the moment, I only have simple games like Match, Concentration and Hangman, for example. The goal is create more complicated, and enjoyable games, so it's possible to fire up an app and play games that'll help you practice the most common words. You can see pics on my site: http://h4labs.com

DuoLingo is probably the best thing if you're starting from scratch but I feel like something is really missing to help people maintain what they've learned.