For the next three hours only, Amazon has the Samyang 14mm f/2.8 IF ED UMC for Canon EF for $269.99. This is $50 off the regular price of $319 for this sharp, ultra-wide angle, manual-focus lens.
This lens is designed for full-frame cameras and one of its best uses is for astrophotography. Read a great (astrophotography-focused) review of this lens.