Here are some ones that come off the top of my head... I don't know too many programs that go on throughout the year (other than maybe city year or teachforamerica (but its a stretch to call those enrichment programs). All the UC's have some sort of research fellowship for summers too. Here are some links to some programs im thinking of...
http://www.shpep.org/ ,
http://www.harbor-ucla.org/family-medicine/community-activity/summer-urban-health-fellowship/ ,
http://medschool.ucla.edu/current-ucla-prep ,
https://medschool.ucr.edu/pipeline_programs.html
Some of these programs as you'll find, look to be designed more for pre-med students. However, they actually apply nicely for pre-dents also ( I can attest to this). At the end of the day, we're all healthcare providers trying to have patient centered care, reducing health disparities, and championing preventative care.
Hope this helps!