The Inkwell is a sketching kit made just for app- and web designers. It's got the essential tools one needs to put together an awesome UI wireframe:

• Hand-crafted bag
• Copic C-1 marker
• Copic C-3 marker
• Micron 05 pen
• Sharpie fine point marker
• 2 Sharpie ultra fine point pens (1 red, 1 black)
• 3 Post-it pads
• 250 Architecture dots

The kit is currently on sale for $49/each as of this writing (normally $69). They also take bulk orders.