Sd is a checker mover that works with all programs up to C4. It even covers asymmetric choreo. Most exciting: It is for free. :-)
Lorenz Kuhlee's WinHash is a very comfortable checker mover program for Windows that covers dancer's animation and has a user-friendly interface to edit and add on new figures or formations.
Vic Ceder created the Ceder Square Dance System. A checker mover and database that can operate with various numbers of dancers (2, 4, 8, 12), asymmetric choreo and is able to create small sequences of choreo. Vic also created SQROT, a program for writing computer cards for dancer rotation.
Vic also offers an online Getout Finder. So when you are on stage with the dancers waiting and you do not know how to bring 'em home, pick up your communicator and click on the link above. ;-)
Those are the programs I find most interesting, but there is more.