I am going on a cruise to Greece and Italy this summer, I really wanted to buy a new Canon lens for my t2i but i am confused whether to buy the 17-40 f/4.0 or the 15mm 2.8 fisheye. Any help?
Those two lenses are going to give you significantly different results. The big thing to note is that the fisheye lens is, well, a fisheye. It'll create severely distorted images that can be "straightened" using software but it's going to have a distinct fisheye look.
It doesn't look bad, but it's not what one would usually choose for a walkaround/vacation lens.
On the other hand the 17-40 f/4L is a well respected lens that covers a good wide-angle zoom range. f/4 should be fast enough for outdoor scenes and well-lit indoor scenes as well.
I'd say unless you're specifically looking for the fisheye look, go for the 17-40.
Without knowing what other lenses you have it's difficult to say what you should get, but if you want to take holiday snaps you want to take as versatile a lens with you as possible so the 17-40 is probably a better bet than the fish eye.
Personally I have an 18-200mm lens which I use when I'm on holiday because it means I don't have to take any other lenses with me. I also carry about my 50mm 1.8 because it's awesome.
Unless you know you need a fish eye then you probably don't need a fish eye, you might be able to get similar a effect by getting a fish eye filter that screws onto one of your existing lenses for an awful lot less money.
I've been to both Greece, and Italy and I can say with 100% certainty that you will want the 17-40 over the Fish-eye. Of the two, the 17-40 is more versatile, and given that the T2i has decent enough high-iso capabilities, the overall usefulness of the 17-40 will easily compensate for the 1-stop light loss.