The Photographers' Gallery (
http://www.photonet.org.uk/) is running a 'Street Photography Now Project'. They set weekly challanges and you can submit one photo via a Flickr group. Detail about the project can be found here:
http://streetphotographynowproject.wordpress.com/about/Below an excerpt of this projects objective:
About the Project
Street Photography Now Project is a collaboration between The Photographers’ Gallery, London and Sophie Howarth and Stephen McLaren, authors of Street Photography Now (Thames and Hudson).
Each week from 1 October 2010, a leading contemporary street photographer will issue a new instruction, written to inspire fresh ways of looking at and documenting the world we live in.
Over the following six days, photographers around the world are invited to upload one photograph in response, to a special Flickr Group. After six days the next instruction will be issued. See the Take Part section of this website for more details on how to contribute.
The Project will run for 52 weeks, and you can join in at any time. The aim is to build a global community of photographers exploring the rewards and challenges of documenting public life. All photographers, including those who contribute to the Instructions, will be encouraged to comment and respond to the images posted to the Flickr groups.
Though not a competition, at the end of the Project one photographer will be chosen who has made the most outstanding contribution to the project across a number of weeks. They will be awarded £1000 of Thames & Hudson books and have their work displayed on The Photographers’ Gallery digital Wall for All.