Pencil is great, you should definitely give it a go if you need to do any diagrams or GUI mockups. I've been working on a prototyping app for a while (https://

Pencil is great, you should definitely give it a go if you need to do any diagrams or GUI mockups. I've been working on a prototyping app for a while (https://www.ux-app.com https://www.ux-app.com). It's more geared toward interactive prototypes. If anyone has any feedback about ux-app then I'd be keen to hear about it :)