I have a Nikon D5000, and am looking to rent a wide angle lens for a day. Any suggestions as what should be a good lens to try out?
The two main WIDE options for DX are the Nikon 10-24mm f/3.5-4.5 and Tokina 11-16mm f/2.8. Since the D5000 doesn't have an autofocus screw the Tokina would have to be focused manually (easier with the depth of field available at these really wide angles). I have the Tokina for my D90 and it's a great lens.
It depends how wide you want to go and whether you have a full-frame or cropped-sensor body, but you probably can't go wrong with the highly-praised 14-24mm f/2.8
Tokina 11-16mm f/2.8 is a wonderful lens. Also you could try the Sigma 10-20 f/4-5.6 . Then there is the Nikon. But personally i prefer the Tokina just because of the f/2.8. Also you could consider a couple of fish eye's. They have a wonderful effect. Maybe the Nikon 16mm f/2.8.
The Sigma 8mm is considered one of the best wide lenses you can shoot. All you have to do is figure out how to keep your feet out of the frame.
The older (than the 10-24) Nikkor 12-24 f4 DX could also be an option. I own this lens and am very happy with it (D80 and D7000). It is tack sharp.
I have never compared it with the 10-24, but from a little (very little) Google research, the 12-24 might be the slightly better performer and of better build quality. The latter may not be of interest to you, though, since you'll be renting it.
The Sigma 8-16mm lens is excellent and really sharp - it's not a FF lens, but should work fine with the D5000 1.5x crop factor.
Sigma also makes a 12-24mm FF lens if you think you might get a larger body in the future, and they also make a 10-20 mm lens - though I don't think that lens is as good as the 8-16mm lens.