I'm quite a new user, so it feels somewhat inappropriate for me to post this (I kinda thought it would have appeared already ^^). But as a member of the Stack Exchange network who just read Jeff's blog post that he was leaving the SE network, I thought I should say something to thank him for all the work that I've seen him put in so far.
So thank you, Jeff, for helping to create what is, to me, one of the best sites on the internet! (and the corresponding network of sites that sprung up along with it) It will be a shame to never wonder again whether you sleep or why your rep is so high on meta, but I hope that in exchange you will have more time to spend with your family.
If it's okay with the mods, then maybe we can all use our posts to thank Jeff!
Edit: As was suggested in the comments, I'd like to add to this discussion by asking members of the community how we can pay tribute to Jeff, and make this, as a goodbye card, a really nice way to wish him well for the future. What would be something good for us to give to a man who has given so much to us?