This list will change as pitch contests come and go (and please feel free to add your own below). Also, please let me know if the links are outdated, so I can keep this post as accurate as possible.
Here are the major Twitter pitch contests running:
For an ongoing calendar, see Meg LaTore's website at iWriterly
Please make sure to research the guidelines and genres for each pitch contest because they are all different and some only accept certain age groups/genres.