I"ve got a Tunze Automat Calcium reactor. It's internal, i hang it inside my sump. VERY small footprint and has had no problems supplying calcium to my tank.
http://www.selectaquariumsupply.com/...eactor,/Detail
If i had to do it again I would go with external because of maintenance being a bit easier if you can see it. But... i add calcium once every couple months, so that's a pretty small complaint.