Toilet Finder is a free app that shows you where to find public public toilets using your iPhone or iPod touch.
The app detects your current location and displays toilets in the vicinity, in both map and list formats. Click on one of the toilets and Toilet Finder will give you more details about the facilities, including the distance, disabled access information, and an overall rating based on feedback from users who have been there. Annoyingly, Toilet Finder doesn't include a search feature. This means a lot of map scrolling, if you're trying to find toilets a bit further afield.
The database of amenities in the Toilet Finder app is far from complete, and the developer has a lot of work to do to for the app to become a comprehensive reference for toilets in all areas. That said, there is a feature for reporting a toilet, so with user power the number of WCs covered should improve.
Toilet Finder also includes a news section, where you can view photos of great toilets from around the world, which is worth having a flick through (perhaps even to pass some time while you're in the toilet!)
Toilet Finder is a handy travel tool to stop you getting caught short while you're out and about. It does need more bathrooms adding, though.