microsoft is hosting a contest for ie7 addon development. all you have to do is create an interesting addon to submit and you could win:
- $2500 plus some tickets to MIX07
- $2000, $1000, $500, Zune
to learn how extensible ie7 is and some starting points, check out the ieblog article from ericlaw.
here's more information from pete on the contest as well.