Not every library can afford Discovery Streaming, Safari-Montage, or a large collection of DVD's to support the school's curriculum; YouTube and TeacherTube offer some alternatives, and it's amazing what you can find there, but it takes time to search through all the funny cat videos. If only someone would do the sifting for us.
Fortunately, there are a few good sites that do just that..find online, free documentaries--often from such venerable institutions as A&E and the History Channel, and then collect them into one easily searchable site. A few of my favorites below. (I'm going to break my end-of-the-post link experiment for this, as it makes more sense to put the link with each entry).
Documentary Wire: This site offers a wide variety of topics, ranging from history to science to psychology and literature, many from Nova, the History Channel, etc. The database is searchable by topic or key word. The site presents each video on a blog-like page, with a brief text introduction, followed by the embedded video.
Free Documentaries Online: Also offers a wide, though more eclectic, collection. It has the added bonus of allowing you to download the videos, avoiding the broadband issues with streaming, which will make your network guys happy.
Top Documentary Films: Excellent, excellent collection of over 1200 documentaries, including docs from the Media Education Foundation, among others. Aside from category and keyword search, the site also includes a useful browseable list of films.
Snag Films: While they offer some good documentaries, I don't find this site's design as easy to search. There's a clickable tag cloud, an A-Z list of films (but with 1,350 documentaries, who wants to go through that?) or you can search by topic/channel. Neither of which I found all that useful. I also think the viewing interface isn't all that intuitive. Still, a good collection worth looking at.
Freedocumentaries.org: Like the above, this site offers a good collection. Click on a given documentary, and you'll land on a page with the documentary's trailer and a description. If you want to view the documentary, you can choose the size you want to "download" to your computer. These are not streaming, so clicking on large--which you would want to use to show to an entire class--can take forever to load. Plan ahead! This site includes such mainstream documentaries as Super Size Me, Bowling for Columbine, and Eyes on the Prize.
UPDATE: One of my old students from Turkey pointed out this site: Documentary Heaven