Realtime location tracking Poll (I'm sure they'll introduce WebHooks soon) the Vodafone API and push realtime updates to a map.
A better Betfair Ok, you probably won't be able to build an app that's better than betfair. But using our API in conjunction with Betfair you'll be able to build a realtime betting system. The questions is: can you get realtime data from Betfair? Things like realtime odds?
Multiplayer Hangman A simple turn-based game. Take the ideas that Dan from Pearson spoke about and use Pusher to push those results to multiple users.
Realtime context sensitive advertising Advertising tends to be pretty static once an application loads. Why not analyse how a user is using an application and dynamically push different adverts to their app… using Pusher :)
Any questions please give me a shout. I'm @leggetter on Twitter.