Some Challenges I've found in my travels ... I have them sorted by Days of the Week, then Monthly, then Miscellaneous challenges that you'll have to investigate on your own.  If you can't find a challenge to fit your project, well, I'm not sure what to say, lol ...

Monday Challenges:
Aurora Wings -
Bunny Zoe's Crafts - (monthly)
Crafting from the Heart - (all challenges are "anything goes")
Crafty Cardmakers & More -
Crafty Sentiments Designs -
Digi Galore Challenges - On summer holiday (still as of 9/24/14)
House of Cards - (monthly)
House Mouse and Friends -
Love to Create Challenges -
Love to Scrap ! -
Mo's Digital Challenges -
Mojo Monday - Card Sketch challenges
Outlawz Challenges - Monday Greeting (Must Have a Visible Sentiment; theme changes weekly)
Papertake Weekly Challenges - (Every Monday)
The Pink Elephant - (Every Monday)
Simon Says Stamps - (Monday challenges)
Sketch and Stash Challenge - (every fortnight, or 2 weeks)
Stamps and Fun = Creativity - weekly
Through the Classroom Door - (Every Monday)
Wild Orchid Challenge - all projects must contain at least ONE WOC product

Tuesday Challenges:
Bearly Mine Designs - (Weekly)
Card Making Mama's - (First Tuesday's)
Clear it Out Challenge -
Colour Q -
Creative Card Crew -
Deep Ocean Challenge Blog - (every 2 weeks)
Fan-Tastic Tuesday Challenge Blog -
Just Inspirational - (Monthly)
Magnolia-Licious Challenge Blog -
My Time to Craft Challenge -
Oldie but Goodie Challenge Blog - (every 2 weeks)
Outlawz Challenges - Tuesday Color - palette given each week
Paper Take Weekly Challenge -
Party Time Tuesday's - (Every Tuesday)
{PIN}spirational Challenge -
Sassy Cheryl's NEW Facebook Challenges - You'll need to join first
Tiddly Ink - (Every other Tuesday)
Tuesday Morning Sketches - (weekly)
Tuesday Throwdown -

Wednesday Challenges:
7 Kids College Fund -   (Every other Wednesday - you need to use product they carry in store)
The ABC Challenge - (1st and 15th of every month)
Aud Sentiments - (MUST include a sentiment!!)
Color Throwdown -
Crafting By Designs - (Plus Christmas challenges ever month til Christmas)
Crafty Creations Challenges - (Weekly)
Crafting for all Seasons - (every fortnight - 2 weeks in USA-speak)
Crafting With Dragonflies - (every fortnight - 2 weeks in USA-speak)
Creative Inspirations -
Divas by Design - (Every Wednesday)
Mo's Digital Pencil Too Challenge - (every fortnight - 2 weeks in USA-speak)
Elphine House Australia - (Monthly - MUST use an Elisabeth Bell image)
Fab 'n' Funky Challenges - (Bi-weekly)
Inky Impressions Challenge - (weekly)
Little Red Wagon - (weekly)
Marvelous Magnolia Challenge - (weekly; they have Beginner, Intermediate & Expert levels)
Outlawz Challenge - Wednesday Sketch (new card layout each week)
The Paper Shelter Challenge - (weekly)
Penny's Paper-Crafty Challenge Blog - (weekly)
Simon Says Stamps - (Wednesday challenges)
Stampavie and More - (every 2 weeks)
Whiff of Joy Challenge - (every 2 weeks)
Whimsy Stamps Challenge Blog - (weekly)

Thursday Challenges:
4 Crafty Chicks Challenge Blog - weekly
CAS(e) this Sketch - (weekly, clean and simple cards)
C.R.A.F.T. Challenge - (weekly)
Crafty Ribbons Challenge - (every 2 weeks)
Creative Knockouts - (weekly)
Meljens Design Team and Challenges -
Milk Coffee Challenge -
Outlawz Challenges - Thursday Twist - Anything Goes projects with a new twist each week
Saturated Canary -
The Sugar Bowl - Inmages by Sugar Nelli - Funky Kits
Southern Girls Challenge -
Sassy Studio Design Challenge -

Friday Challenges:
Art by Miran -
Brown Sugar Challenge -
Card Makin' Mamas Challenge -
Chocolate, Coffee and Cards Challenge -
Crafty Catz Challenge -
Creative Fingers Challenges -
Creative Friday Challenges -
A Day For Daisies Challenge Blog -
Delicious Doodles Challenges -
Dilly Beans Friday Challenges - Require Dilly Beans images for submissions
The Friday Mashup Challenge -
Fussy and Fancy Challenges -
Kitty Bee Challenges -
Lia Stampz Challenges -
Outlawz Challenge - Friday Color (using images colored with markers, pencils, etc.)
Sister Act Card Challenges -
The Squirrel and the Fox Challenges -
Stamp and Create with Magnolia - MUST use a Magnolia image
Tilda's Town Challenges -
Time for Magnolia Challenges -
Wags 'n Whiskers Challenges -

Saturday Challenges:
A Gem of a Challenge - (Every Saturday)
All Dressed Up Challenges -
All Sorts Challenge Blog -
Crazy 4 Challenges -
Creatalicious Challenges -
Fantasy Stampers Challenge Blog -
Freshly Made Sketches -
Kraftin' Kimmie Stamps!! Challenges -
Make it Colorful Challenges -
My Sheri Challenges -
Outlawz Challenge - Saturday Paper ala Mode
Penny Black Saturday Challenge - (Monthly)
Sketch Saturday Challenges -
Sweet Stamping Challenge Blog -
Time for Magnolia Challenges -
Totally Gorjuss Challenges -

Sunday Challenges:
CAS on Sunday's Challenges -
Cheery Lyn Designs Challenges -
Crafting When We Can Challenges -
Glitter and Sparkle Challenges -
Hobby Cutz Challenges - (taking a break from 10/6/14 for a bit ...)
Kenny K's Krafty Krew Challenges -
Lacy Sunshine Challenges -
Lil Patch of Crafty Friends Challenges - Starts January 2015
Lovely Hangler or Sweet Magnolia Challenge Blog -
Magnolia Down Under Challenges -
One Stitch at a Time Challenge Blog - Use Stitchy Bear images = unlimited entries; use other than Stichy Bear, limited to 3 entries per challenge
Outlawz Challenges - Sunday CAS (Clean and Simple, with twists each week)
Pile it On Challenges -
Robyn's Fetish Challenge Blog -
Scribble and Scrap Blog and Challenges -
Sentimental Sunday's Challenges -
Sketchy Colors Challenges -
Sunday Sketch and Stamp Challenges - (every 2 weeks)  as of October, 2014, not a lot of entries, so chance of winning = greater
Sunday Stamps! Challenges - (every 2 weeks)
Totally Tilda Challenge Blog -
Whoopsi Daisy Challenges -

Monthly Challenges:
Bearly Mine Challenges -
Crafts Galore Encore Challenges -
Catch the Bug Challenges -
CLP Challenge Blog -
Doodle Pantry Digital Doodle Challenge Blog -
Elisabeth Bell's World Challenge Blog -
Emerald Fairies Challenge Blog -
Just Some Lines Challenges -
Ladybug Crafts Challenges -
Lili of the Valley - Ideas to Inspire Blog -
Natalie & Amy's Challenges -
One Sweet Challenge -
Sketch 'n Die Challenges -
Stamping Sensations Challenges -
The Sugar Bowl Challenges -

Miscellaneous Challenges - you'll have to check monthly or random here ...
Everybody Art Challenge - (Weekly)
Kit and Clowder - Facebook site - you'll need to join
Kit and Clowder - Ning site, again, you'll have to join - WONDERFUL coloring classes available
Love to Create Challenge Blog
The Outlawz Challenges - (7 different challenges each week)
Paper Makeup Stamps - (Weekly)
Paper Playful - You'll have to check this out for yourself ;)