I just signed up for a free account at Gliffy, which bills itself as a web based diagramming program that feels like desktop software. It works a lot like Visio.
Signup requires an email address, so I probably won’t use Gliffy with my middle school students — which is a shame. I continue to struggle with how to handle signups that require email addresses for my youngest students (approx. age 9). But that’s a separate issue.
On to features I like in Gliffy:
- share a drawing with collaborators
- publish drawings to the web easily
- loads fast (it’s Flash based)
Below is a sample drawing I whipped up in about three minutes (and that includes the “publish to web” bit).
I bet that Google buys Gliffy to add to the Docs and Spreadsheets tools they already have.
Megan Hayes-Golding 
Solving the email issue
If your school has google apps gmail, set up a catch all email address, give each child a fictitious email address under your schools domain ie pupilname@myschool.com.
The mail will be forwarded to the catchall account and your students will be able to complete the registration process. Quite a bit of work but has the desired effect.