Chinese (self-made, just adding words as I go), geography (The Ultimate Geography deck, plus customized decks for US, Canadian, and Australian political geography), the NATO phonetic alphabet (self-made), the periodic table (self-made + shared), and US Presidents (self made + shared).

I don't plan to add any more for a while because between Chinese, Geography, and the periodic table it's 40+ minutes a day already - but once the geography and periodic table decks have run out of new cards and settled into a dozen or so a day I might look for or create a nice piano deck. Maybe some more geography related ones as well.