I tried SpiderOak for a while last year. While there's a lot of overlap, Dropbox and SpiderOak are different in nature: Dropbox is a simple-to-use file sync service with limited online backup features, while SpiderOak is a complete online backup service with harder-to-use file sync features.
Balsamiq's "Mockups" tool is ideal for throwing together the straw-man, discussion point, come... well mockup, for application development. It's the perfect "electronic napkin". Quick. Intuitive. And if you're a scribbler, doodler, or back of the envelope kinda guy then this is the perfect app.