As an amateur photographer who likes to compete with my work, I have been a member of a few websites where I can compete with my photography, or show it off.
- http://Photosig.com has a very large member base that allows me to present my work to a group of people and have them critique the work. Those critiques often point out areas in my photography where I could improve.
- http://www.Photofriday.com is a weekly challenge to keep me shooting a variety of subjects and often has 500 or more entries for the challenges that start each week on you guessed it, Friday.
- Another website the I frequent has both weekly and monthly assignments; http://www.fredmiranda.com (look in the forums, free registration necessary).
- DPChallenge.com is another site that offers weekly challenges. DPChallenge offers both free challenges (requires free registration) or member challenges for paid members of the site.