Sewing Stash is the sewers look-up book to all things sewing on the web. Through this website I accidentally came across http://tipnut.com/free-apron-patterns-tutorials/ which is as awesome as it sounds. It gives the most comprehensive look at sewing aprons that I have seen.
Freeneedle's motto is " we still do the searching for you." Type in whatever type of pattern you are looking for and Freeneedle will find it!
COPA is the University of Rhode Islands' commercial pattern archive. Dating back to the 1850s, imagine what you can find!