There's a great site called Mapnifiscent. You put in a location, it calls up the Google Maps GTFS API (probably), and it tells you where you can get in n minutes. What if you have a bike? If you have a bike, you can change the settings and say "I have a bike!" and it will factor that in to the equation. (One minor point of contention: it assumes 3 mph straight-line for walking, but just 6 mph for bikes. 3 mph in a straight line for walking seems quite fast, as it probably equates to 4-5 mph when roads are taken in to account. 6 mph seems slow, although when I calculated Hubway speeds it is actually a pretty good estimate for straight-line urban biking speeds.)
What if you have a Bike Share fob? Well—that's anyone's guess. A great new feature would be for Mapnifiscent to scrape the XML feed off of bike share sites (for instance, here's Boston and DC) and plug locations in to their formula. It could then offer an option "are you a Bike Share member" which would allow estimates based on combined transit-bike share-walking trips; and show how bike sharing can enhance transit by expanding access. Because, let's face it: you don't always have a bike with you. And sometimes it's not allowed.