Boston Marathon - A must do for every runner, especially if you're from New England.
New York City Marathon - I'm not much of a city person, but big city marathons are pretty cool (other than the logistics).
Pikes Peak Marathon - I have always said I would never run this, but after having to turn around at mile 10 of the 2008 Ascent and run back down I decided that racing down the Barr Trail might be fun.
Western States 100 - The most prestigious ultra in the country and it covers some awesome terrain.
Leadville 100 - I've been through Leadville once and wasn't too impressed, but the race course looks beautiful.
White River 50 Miler - awesome course, great scenery and usually pretty competitive.
Dipsea Trail Race - A classic trail race in California with a unique age/gender handicapped start.
La Ciaspolada - The biggest snowshoe race in the world, in Italy (6,000+ finishers!)
Sierra Zinal - one of the European mountain classics
AMC Hut Traverse record attempt
Ski the American Birkbeiner XC ski race in Wisconsin
Ski the Tuckerman Ravine headwall - will probably never happen unless I work on my downhill ski skills and grow some gonads
Ski from the Mt. Washington summit to Pinkham Notch Visitor Center
Hike the length of the Appalachian Trail in NH as a thru-hike
Ski from my house to Canadian border on snowmobile trails (~200 miles?)
I am sure I should have something related to road or mountain biking, but I can't think of anything yet. Feel free to add any suggestions or post your list on your blog!