Multicolr lets you mix up to 10 colors and picks images matching the chosen color palette. Very cool! My favorite
Tiltomo is a lot of fun to use as well. This tool allows you to search by tag, color / texture and theme / subject. It’s fast and uses two data bases for images.
Findr Searches by tag and gives many alternative tags for your search
PiciShare is a Flickr search engine. Your search is chosen by tags or full text; you can limit the number of images per page and sort results by importance, date added or popularity.
Airtightinteractive displays images based on your tag choice and also recommends more related tags:
Compfight is a tag-based search, but it lets you set the Creative Commons filters. This is a great option.
FlickrStorm searches by keyword and /or by Creative Commons. It also has a neat flash-based interface
Flyr searches within geotagged photos and adds Google map links to each search result.