Northeast Cycling - Doug Jansen's site is a fantastic resource for climbing in the Northeast. The focus is on the larger hillclimb type of climbs (like Mount Washington, Ascutney, etc.) There will be some overlap between our sites, as he also provides information on average and max gradient. His gradient maps are made somewhat differently, and as such have slightly different emphases. He also provides great information on selecting the right gears and estimating your finishing time for climbs such as Mount Washington. His blog also makes for fascinating reading.
Climb By Bike - A nice but sometimes infuriating site with a european focus. I don't find the interface terribly intuitive, and the pictures and ads sometimes make the site run painfully slow.
Salite.CH - This Swiss site has some other hills, but makes similar gradient maps to climb by bike.
My Blog - you can find some of the same info here, as well as some discussions about my own fights with gearing and setup and whatnot. Boring as hell. Don't go there.
Chapman Cycles - Sadly, Circle A is closing up shop after a good long run. Brian, who built my frames, has been on his own for a couple years now, and is producing beautiful frames in Pawtuxet Village. And Brian can also beat you at George St.