LegUp is an open source high-level synthesis tool being developed at the University of Toronto. The LegUp framework allows researchers to improve C to Verilog synthesis without building an infrastructure from scratch. Our long-term vision is to make FPGA programming easier for software developers.
Download Source
LegUp 2.0 (Dec 19, 2011) | Linux required
Here's a longer version of the talk Andrew Canis gave for our FPGA 2011 paper: LegUp: High-level synthesis for FPGA-based processor/accelerator systems.