Azrin & Foxx's applied behavior analytic approach to potty training. Need to get the book! EDIT: potty-trained my 26-month-old daughter using this technique. A little technical and rigid, but well worth it (It took us about 2 days)
Free and lots of features like a timer to remind us it was time to try again and short little games with the kids' favorite Disney characters that they got to play when they were successful. SO thankful we'll only go through this one more time!