Although I’m currently doing marketing for the good folks at FreshBooks, I came from a tech background. Two of the bigger things I have worked on in my life are wikis and Scalable Vector Graphics (SVG), a XML standard clone of Flash. I have an undying love for the <g> tag, which does not need your prying questions.
So, I was pleasantly surprised to find amongst our customers one, Gliffy, a company I have been watching and admiring for a long time. Joining other Office 2.0 apps like Google Docs & Spreadsheets, Gliffy is the web 2.0 version of Visio. Gliffy lets you create a diagram, share it with others, work on it together, and publish it when complete. All versions are kept. Everything is entirely web-based and so you can access and create diagrams anytime, anywhere. And it looks and feels like standard desktop drawing tools, which I found makes it very, very easy to learn.
Even better, they have released a plugin for Confluence, the current market leading enterprise wiki. And they export SVG! Too awesome. How I would have loved to have Gliffy at my last gig while negotiating message sequence diagrams with our client.
Anyway, checkout the Examples section on the Gliffy website to see what you can accomplish. Some of the diagrams are quite impressive. Gliffy has a natural fit with information architects, software developers, engineers, and architects.
Update: Gliffy suggested I mention their demo, which is très cool.