I run a web development company in the UK[1]. Last weekend, we participated in Leedshack[2], and we build Barebones, which is a really simple wireframing tool for the iPad.
We spend a lot of time in meetings sketching in our notebooks, and often these sketches are a valuable artefact long before 'traditional' wireframing even enters the process; we wanted to replicated the speed and simplicity of 'back of a napkin' sketching, and combine it with really simple sharing (anything is better than taking a photo of a sketch with my iPhone and emailing it to a client!)
It's a HTML5 app, which uses <canvas> and the Dollar Gesture library to recognise shapes drawn on the screen and snap them to a 960 grid.
Our intention was for it to be so quick and simple to use that is can take the place of a quick pencil sketch in front of a client. It really is very simple to use, and we limited the gestures to just three very distinct movements, and with a little practise, it's actually very quick and accurate.
As I mentioned in the title, we built this in 24 hours, so it lacks quite a bit in polish, however all the guys on the team would love any feedback from fellow HNers.
[1] http://www.testled.com/ [2] http://www.leedshack.com
Edit: A lot of people will be reading HN in a browser; Barebones does work in any reasonably modern browser, and it works with mouse events, but it really doesn't feel 'right'. Try it in an iPad if you can.