Good question! I've been using MapMyRun but I'm not happy with it. About 1/2 the time it messes up my pace, which is correct on the Garmin.
I think I saw that we can download heart rate data from polar devices.
How can we suggest this to the developers of this site?
Click on "My active" then on "Feedback" it is a short one page survey.
Tell them here to add the ability to download Garmin Forerunner data to our training logs.
I use SportTracks (shareware) software to analyze my Garmin data. The software (or one of its free plugins) will export your data into several different "standard" formats that you may then be able to upload to Active. If you are so inclined, it might be worth looking into.