Soydios, you should add the Sigma 24-60mm f/2.8 EX DG to the list of third party lenses. Here is my impression of the lens after having it for a day:
Build quality is excellent; it's not quite as bombproof as the Nikkor 17-55mm f/2.8 I used last week, but I wouldn't put it too far behind in terms of build quality. It is noticeably better than the Tamron 17-50mm f/2.8 in terms of build quality (a friend of mine has the Tamron).
The lens is IF and does not rotate on focus or zoom; it also has a zoom lock at 24mm as well as an aperture ring so it's compatible with older cameras and accessories like extension tubes. The lens is shortest (about 3.5") at 24mm and extends about 2 inches at full zoom (60mm).
It focuses with the camera's built in AF motor (so D40 and D60 are out), but AF is quick and accurate.
The lens is 24-60mm, which is a less useful range than 18-55mm. But...the fast aperture and great optics make it worth the tradeoff IMO. It is compatible with full frame 35mm and digital cameras.
Image quality on mine is great; sharp even at f/2.8. I posted some sample pictures in another thread, but if you want more I can upload additional ones.
It comes with a nice Sigma "EX" carrying case and lens hood, as well as 4 year warranty.
Best part: the lens only ran me $235 shipped from Cameta Camera, brand new with 4 year Sigma warranty. It even came with a Hoya 77mm UV filter, cleaning kit, free 1 year subscription to PC Photo, and 50 free prints! This is dramatically lower than Sigma's $570 MSRP, and a steal for the build and optical quality this lens offers.