The Back Up Box
Professional DJ's keep their playlists on a laptop computer, which is hooked up to large speakers; also a possibility to consider. The advantage to using a laptop is the ability for an unlimited number of songs on your playlist, and it is easier to arrange or modify the order of the songs.
The Dance Floor
If you are having your reception outdoors and want to dance I came across the perfect solution while cruising blogs. A DIY project for the guys.
Laura posted this on her wedding blog and included complete instructions with pictures.