I'm also a huge fan of Peldi - I love that he personally responds to emails and tweets and seems to actually give a shit about his customers. (Very refreshing after my stint at Google, which is severely lacking in that department :).

I also love Balsamiq - I use it both to prototype web/mobile apps and also for diagrams in presentations (http://client-side-apis.appspot.com/). The sketchy feel makes the prototypes feel more flexible, perfect for encouraging iteration.

I've been using myBalsamiq (the online version) for a while, and though it is Flash-based, and Flash is the devil, it works pretty well and I recommend it.