Hunting the Zodiac is the 2007 film by John Mikulenka documenting various people's investigation into the mystery of the Zodiac. The material is a bit dated, e.g. there's a heavy emphasis on Arthur Leigh Allen as a suspect. Nonetheless, it provides an interesting glimpse into the Zodiac community and also provides some valuable insight from then-San-Francisco Homicide Inspectors Mike Maloney (who sadly passed away in 2007) and Kelly Carroll. You can read more about the documentary at its website.