I think the most recent addition (I haven't seen any of the National Park coins yet) is the new Indian dollar series, with Sacajawea on the obverse and various designs on the reverse. I think the first of these just came out. I get a roll of dollars every week, save one (if I don't have it already) and spend the rest...i figure that I'm doing my part to make them circulate.

Here is the US Mint site to get a subscription to the Park quarters. The plus-up is substantial - $13 on 2 rolls.