There's a huge list of monthly challenges on Wikipedia (here) but some of my favourites are:
- NaNoWriMo - National Novel Writing Month - every November, write 50,000 words in November
- April Fools - Set your own target and write that target in April
- FAWM - February Album Writing Month
- Camp NaNoWriMo - Like NaNoWriMo but happens twice during the summer and you can set your own target
- JanNoWriMo - Write 50,000 words in January
And Blogging from A-Z is another one :)
What's your favourite monthly challenge?