A related option to allow entry based on a code, if you don't want to replace your lock, is to put a combination key lock box (sometimes called a "realtor box") somewhere near the door. We have one of these and have been very happy with it. And no batteries that can die!
>>jefftk+(OP)
One threat model with that kind of approach is that once a visitor possesses your key -- even if they return it -- you can't tell how many copies they may have made.