I love writing challenges – it started with NaNoWriMo of National Novel Writing Month were you write 50, 000 words in a month. Nano has a huge community with a forum were help can always be found or an exercise to get you going, and local groups so you get to meet other local writers. It really helped boost my writing and I highly recommend it.
Also they don’t mind if you are a series of flash fictions instead of a novel!
There are many others NaPoWriMo, PoMo and WoPoWriMo which all deal with poetry and PiBoIdMo which is picture book writing. What ever you are into writing there will be a challenge out there waiting for you!