I just saw this on Doug Mahugh's blog and it's really cool. Stephane Rodriguez has built a tool that allows you to view a diff of two Open XML files. I was actually bugging a number of folks to see if they would build something like this and it's awesome to see someone outside of Microsoft stepping up and pulling this together.
I've seen more and more momentum from folks in the community building solutions and now that the standard has solidified and is in it's final draft form I think we'll see a huge increase developers taking advantage of the new formats. If you have a solution you've built, I'd love to hear about it. Feel free to send it on via the comments functionality on the blog or you can click here to e-mail me directly.
The best site though for collaborating and sharing ideas around the Open XML formats is of course the openxmldeveloper.org community. It's pretty cool to see all the folks that have already delved into the earlier drafts of the standard and have positioned themselves to be the leading experts in what will become one of the most widely used file formats in the world.