I don't know if you would be interested in this at all, but I use Planbook. It's not a web-based application, but you can publish your lesson plans to any server. I think it's one of the best software programs I have found.
I am the founder of sharedschool.com, an online lesson plan management tool that uses a correlation engine to enable teachers to discover others' content simply by searching their local standards. We are still in our alpha release and the site needs content - but if you do an advanced search (http://sharedschool.com/advancesearch.php) for Massachusetts>5th grade>Math>Number Sense>->5.N.12, you'll see some correlated content from Indiana. This is just an example of how, working together, we can save teachers time and money. There is a 90 second video on the site but let me know if you want a demo too.