As usual, 3 monthly ideas for free:
Auto Repair MaShup
Google mashup, for auto/insurance, with OAuth support for login, where users create their review of auto repair shops. A Yelp for auto-repair shops. Maybe integrated with Foursquare: free check-ups for 3 check-ins 🙂
Cluedo for Facebook
A Facebook application challenging you to find who is the hidden friend, based on clues gathered on each profile. Probably would fit nicely as a brand contest.
Mixing Twitter with product placement: work with a soap opera or entertainment show, and get to know in advance the place on set of the product placement, or the time within the episode. The first 10 people who @ reply the brand account with the exact location on the set of a product (or the elapsed episode time), get that product for free.
That’s it. Now, go make it real.