The best of the ultra-wide zooms is the Tokina 11-16mm f/2.8, a truly amazing optic - that's going on £500 new, probably mid-£300s used. Sharp all the way across the frame, it really does the D7000's sensor justice.
The Tokina 12-24mm f/4 and the Nikon 12-24mm f/4 are brilliant also; the Nikon edges it on the build quality front but new it is expensive.... you'll pick up used copies of both for about £250 and £450 respectively. I used both on a D2x and they were brilliant. On a budget, the Tokina is the best value ultra-wide on the market I reckon. Buy it used and it's even more of a bargain. This is the kind of thing I used to shoot on my 12-24mm:
Setting the trap... by
Pat MacInnes, on Flickr
A lot of people like the Sigma 10-20mm because it's at the budget end. There are two versions, one is the newer f/3.5 version and there's a variable aperture version too.
I've no first-experience of the Tamron 10-24mm, but it never seems to be mentioned with real enthusiasm. There's also the Nikon 10-24mm that can be had cheapish on the used market but being Nikon, does command a high RRP of about £650.
If you want to have real fun then check out the Nikon 10.5mm f/2.8 fisheye.... one of the sharpest, most bonkers lenses I've used. Size-wise, it's only slightly bigger than a 50mm. I've seen them going for as little as £250 used, which is an absolute steal considering the amazing optics and build quality. And it focusses down to about an inch so is brilliant for extreme close-ups. There are other fisheye's on the market but this is the one that gets the plaudits. This was shot on the 10.5mm:
Fighting To The End by
Pat MacInnes, on Flickr
For a few years I used a Sigma 14mm f/2.8 on my DX bodies. Build-wise it was amazing and sharp as anything, but it was particularly prone to flare (as is the case with ultra-wide lenses). Like the 10.5mm, it focussed down to something like an inch from the front of the lens, which made it seriously useful for putting small objects in the frame. This was shot with it:
Who's afraid of a little spider? by
Pat MacInnes, on Flickr