Here is a list of some of the Photo Challenges I have found across the web. If you know of any additional ones please let me know. 
Many of these challenges start on the day they are listed under and continue through the week until the next challenge is listed.
MONDAYS
Moody Mondays
TUESDAYS
Two4Tuesday
WEDNESDAYS
Lens Day
Who Do You Love?
THURSDAYS
Thursday Challenge
FRIDAYS
Photo Friday
SATURDAYS
ShutterDay
Photo Hunt
SUNDAYS
See It Sunday (theme announced on Saturday)
ANY DAYS
DPChallenge
JorjDotOrg PhotoHunt
Macro Day
Unique’s Photo Challenge (starts on the weekends)
Wordless Wednesday – EVERYDAY
