If you are in the market for a new or used lens, the following lens databases may be useful.

Canon mount:

The Lens Database

Canon EOS SLR camera system lenses

Minolta Sony:

Minolta Sony alpha lens database

Konica Minolta official product site

Nikon mount:

Official Nikon Imaging current lens product pageNikon Lens information (serial numbers, specifications, accessories, and summary)

Nikkor Autofocus lenses database

Pentax K-Mount:

Pentax K-Mount Page

Non-specific:

Lens database at photodoto

I am trying to create a one-stop lens database collections for all SLR cameras. Please let me know if you know a good lens database not listed above.