"Need" is perhaps a strong word, but a matte box serves two purposes: one, to cut down on unwanted lens flares; two, to allow for easily changeable filters in front of your lens. You may not need one; it depends on how you feel about lens flares, whether you're planning on using filtration, and how high-profile you can afford to be if you're on a guerrilla production (a matte box screams "real film" more than perhaps any other accessory, which I suspect is why a large portion of filmmakers run around with one attached: to look legit). Here with a nice, brief video of real-world examples of these issues is accessory manufacturer Cinevate:
Video is no longer available: vimeo.com/31434786
More info on Cinevate's blog.
Companies making matte boxes in the $1,000 or less range (because you can spend a lot more if you go with an ARRI or Chrosziel unit) include Cinevate (of course), Redrock Micro, Shoot35, Light Craft Workshop, Cavision, and Genus.
I have a cheap Indian knockoff that I'll be replacing in the weeks to come. Anyone have a particular model of matte box you love or hate?
DISCLOSURE: Cinevate is a NFS sponsor.