Having come from the world of Web design & gathering client feedback, one of my first efforts on Photoshop was to ship templates for the (now deceased) Web Photo Gallery that enabled viewers to comment on images. Now The Turning Gate offers a much more sophisticated tool for Lightroom:
The Client Response Gallery facilitates communication between the photographer and client following a shoot. The photographer publishes a web photo gallery of images from the shoot, and the client marks images as selects by ticking a checkbox for each image to be kept. Selected images are then submitted to the photographer’s email address as a comma-separated list, which may be copy-and-pasted into Lightroom’s filters to quickly isolate images in the catalog for processing.
Check out the site for many, many more details (e.g. how it works on mobile devices). The tool costs $25.