Here is our updated list of running contests/challenge on Steemit. 

It's not complete, of course. (Which projects did I forgot?)

 The STEEM-POCALYPSE SURVIVAL GAME SEASON 3 is an amazing game managed by @papa-pepper, who also manage WILD & STRANGE CONTEST.

The MEME-CHALLENGE is in its 22th weeks. You can easily roll on the floor laughin if you check the entries. (By @fibra59)

STEEM Cup Series is a contest connected to Nascar car racing. It's managed by @ats-david, who also rules the Steemit Scramble: PGA Tour, connected to the Golf Tournaments.

The  Steemit Photo Challenge is a photo contest. on its 43th edition! (by @jamtaylor)

The SteemTrail communities manage various contest:

The  Art-Trail Contest, organized by me for @art-trail :)
The  50 Word Story Contest, managed by @ogochukwu for the @literature-trail, is on its 16th week.
The  ART-Challenge, managed for  @artquest-trail  by @aksinya, who manage also another amazing art contest, the ARTstyleART, and the Travel Contest!

SpanishChallenge is a contest in spanish managed for the @spanish-trail by @tincho 

ColorChallenge is a permanent project launched by @kalemandra.

A new contest is  The Steemit streetART Challenge, organized by @funkit

There are also the daily thematic photos:

MonochromeMonday - initiated by @old-guy-photos - check all posts here Monochromemonday:
TreeTuesday - initiated by @old-guy-photos - check all posts here TreeTuesday
BeachWednesday - initiated by @uwelang - check all posts here BeachWednesday
SunThursday - initiated by @uwelang - check all posts here SunThursday
LoveFriday - initiated by @liliana.duarte - check all posts here LoveFriday
BeautifulSunday - initiated by @ace108 - check all posts here BeautifulSunday

One of the most successfull contest is Openmic, an amazing musical contest managed by @lucypher both in english and spanish.

There's also a  STEEMIT CULINARY CHALLENGE, managed by @woman-onthe-wing 

By Hive account@timsaid, there is the  The Steemit Crypto Challenge, with a prize of 100Steem.  It's an hard crypto-puzzle to be solved!

Another photo contest is the B&W Photo Contest, managed by Hive account@daveks.

 And I can't forget the amazing Collaborative Art Journey managed by Hive account@everlove!  

Update:

Thx to the commenters, I can add some more contests:

MyPictureDay is a photographic project managed by Hive account@timsaid.

The Steemit Portrait Photography Contest is another photography contest, dedicated to portraits.

 Hive account@jonny-clearwater manages the Pixel Art Weekly, on its 4th week now.

Brainsteem is a mathematic puzzle game managed by  Hive account@rycharde in the  math forum.

I surely missed other contests and challenges: tell me which ones are missing and I'll update! Thx

@paolobeneforti