While surfing the web for something totally unrelated, I stumbled across MileMaven.com which promotes itself with the byline of Earn the Most Miles on Every Trip.
Simply enter the departure and arrival airport codes and it will return all promotions available based on that segment.
Other search options let you enter frequent flyer program, airline code or just retrieve the most popular programs. I tested out one of the promotions (additional 1000 bonus points) and it took me to a valid promotion on the United website.
The search results also provide links to various Flyertalk discussions on related airports / segments. Not a bad tool to keep in your back pocket.
Looks like it also won a honorable mention from the Washington Post.