FlickrCash uses the Flickr API to search by CC license, build lightboxes, and keep a record of licensed photos you intend to use.

Augustine Fou, creator of FlickrCash, tells us:

I created FlickrCash because I found many really beautiful photos on Flickr but could not use them for “commercial” purposes like design work for clients, because there was no way to document I had a license to use it. FlickrCash is BOTH a search/find interface to more quickly find images on Flickr, and also a way to document that you have a license to use a specific image.

Sample of image search (currently only searches Flickr repository):
http://flickrcash.com/?k=flowers

Sample of archived license, available for inspection at any time:
http://flickrcash.com/license/27i8d5sf

With this publicly archived license the image buyer can definitively prove they have the right to use a specific image for a specific purpose — so they can use it for client design work. Both image owner and image buyer are named signatories to the agreement, and an official date/time stamp is obtained from the NIST Atomic Clock to document the exact time the license was executed.