There are several lens tests sites and reviewers available in internet. If you are interested on those lenses mentioned on earlier pages and want to know more about their image quality and other aspects, just make a search...
Some thought s about those sites and test anyway:
1. Do not overestimate those results. Most of the modern lenses are at least decent IQ-wise, giving sharp enough results. The IQ varies with some sample variations, with the camera used for testing and the method used. Most testers use flat test boards as a subject - and get compareable results from that setting. That's ok, and very right.
However, just a few of us use walls or flat boards as a subject for our photos. The world is 3-dimensional... If the focus plane of the lens has some curvature, it shows as softness in the testshots, and in real world photos it may not show at all. Or give a pleasing soft effect in the corners. Or not.
2. Compare different sites and results. The testers and writers are fortunately human beings... with their likes and dislikes.
3. Search for user comments and reviews. Don't underestimate them. Maybe some sample shots? If the results are something you like and are searching for, maybe you have found what you were looking for. On the comment side, the negative comments seem to rise easier than the positive ones, and usually from people that have never used the lens they are bashing. And user reviews are usually very positive... Read carefully.
Also, if the review/comment is about a lens which is wider or longer for what the writer is used to, there may also be a natural learning curve to go... Long teles are long teles and, as that, more difficult to use than "normal lenghts", never mind how good the IS or OIS or whatever shake reduction system is. Focusing is more difficult, finding subject is more difficult... Same goes with the ultra wides; they also need some learning before you know how to use them.
4. There are also pages for older lenses - they can give a hint what can be done with so called legacy lenses. Those lenses can give a really interesting view for photography. They might not be as sharp or contrasty as the new ones, but there are some interesting options.
PS. Now there are even AF-adapters for Canon-lenses, and as far as the testers are honest, they seem to work fine... Means even more options.
Links for some lens test sites:
DPreview lens tests
Some interesting Blogs
For older lenses:
BRANDY MORECASTLE http://brandymorecastle.org/projects/fourthirds/om-on-four-thirds.html
What mount? http://rick_oleson.tripod.com/index-99.html
And of course, camera and lens manufacturers and other official sites