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
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.