Before buying a lens I would like to see what kind of pictures it can take. Which websites let me see all the photos previously taken online by a particular lens (or a lens/camera combo)?
I have used PixelPeeper for that. It is a site which was made specially for this purpose.
For example, if you want to see images taken with the Canon EF 135mm f/2 L lens, just go to Flickr, type in "EF 135mm f/2 L" in the search box in the upper right hand corner of their main page, select "Photos" from the dropdown menu, and you'll get a plethora of images with that lens in the EXIF info.
Most other image sharing sites can be similarly searched by lens.
Another place is dpreview.com https://www.dpreview.com/sample-galleries?category=lenses