You could add an attribute...
Haven't done this in a while, so bear with me if it ain't completely correct:
1. Make sure the door's pivot is where you want the it to revolve around.
2. Go to Modify -> Add Attribute and name the attribute (something like "swing" or "open" or something like that), and enter the values. Minimum should be 0 and Maximum should probably be the amount you want it to be able to open in degrees (if you want it to open to 180°, make the max value 180) and default doesn't really matter...
3. Key the values with the rotate attritube using Set Driven Key (Animate -> Set Driven Key -> Set [Option box])
Assuming you had to door selected, it will be loaded as the Driven object. If not, click Load Driven. Still with the door selected, click Load Driver.
In the Driver window, select you new attribute, and in the Driven window, select Rotate Y. Make sure that both attributes are set to 0, and then click Key.
Set the new attribute to its maximum and Rotate Y to the max you want the door to open, and click Key again.
You should now have a door that opens. To test it, click the attribute you added (the name, not the input box) and drag with the MMB in the work area, and watch your door swing open!