I mentioned a while back NewsJack, which is a great opportunity for new writers looking to break into radio comedy.
Another site worth checking out if you're in the UK is BBC Writersroom:
Responsive, proactive and an open door to writers, BBC writersoom is always looking at new ways to find and champion talent for all BBC platforms.
We know how hard it is to be a writer and we believe in finding ways to inspire and inform you, to keep you across changes, giving you access to commissioners and production departments but most importantly to the skills and and experience of established writers.
We are not here for the short term, we understand how long it can take for a writer to find that first and second commission, but if we think you have talent then we will do everything in our power to support and develop you for the long term. Read more here.
They have open submission times throughout the year for assessing scrips, as well as a guide to writing scripts and an extensive archive of example scripts to look through. they also advertise other writing opportunities to help you learn and develop. As well as their website, you can find them on Twitter: @bbcwritersroom, Facebook and on their blog.
If you're new to scriptwriting, check out my previous post on Celtx, free formatting software.