With the opening of the Intercounty Connector on March 1st, there is going to be a new metrobus from BWI to the Shady Grove Metro Station. Route is numbered 201. It will be free for the first 2 weeks and $5 after that.

Given that it is a longer trip than to Greenbelt, this is probably only of use to people going to Rockville or Bethesda, but it might also help to Woodley Park or Dupont Circle, which are frequent tourist and convention destinations. And it is always good to have more options.