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.
The Fisheye is a lens for special effects. You certainly don't want to buy this as a lens for documenting your vacation.
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 would say for a greater range of possible uses, it would probably be best to get the 17-40 over the fisheye.
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.
You have a cropped sensor camera, I would consider an EF-S lens. These tend to be cheaper and lighter than similar quality EF lenses like the 17-40.
Consider the 17-55, 17-85, or 15-85.
The 17-40 will take sharper pictures than the kit lens, but it doesn't have IS (Image Stabilization) so you may actually get blurrier pictures when you use it, in some light conditions!
If you are looking for a wide angle lens, either to fit more into the frame or to get a wide-angle perspective , you should read http://www.kenrockwell.com/tech/how-to-use-ultra-wide-lenses.htm which covers the subject well (I know, I know, KR this, KR that. But it's a good guide to effective use of wide angle lenses). Also, you should know that the 17-40 gives you only about a 6% wider view than the kit 18-55 lens! So if you think you want, or have been told to get, "a wide angle lens," you should look into the 10-22 or its off-brand equivalents.
If you don't already know that you want the fisheye, you probably shouldn't get it. As others have mentioned, it's a specialty lens, and you should only get a specialty lens if you have specific uses in mind for it.
Have you considered other lenses, like the 17-55?
Finally, you are just going to be shooting things close-up, right? Because neither of the lenses you asked about (nor any of the lenses I've mentioned) will be particularly good at capturing anything detailed more than a couple hundred feet away.