I'm playing with Google Docs Drawings. Not only is the software phenomenally easy to pick up, but like all Google documents it's online. I'm just now starting to understand the profound impact this is going to have on our working culture.

Browser-based: all you need is a web browser to get started; no install, no maintenance, no backup etc.

Collaborative: when you're authoring a document, others can be viewing your work or contributing to it as it's created.