I think the consensus will be that you aren't providing enough information. Maybe there isn't much more available, but you're in a better position to provide it than anyone is to guess.
If a couple of numbers are known that makes it a whole lot easier. One is the head, or difference in height between where it comes onto your property and where it leaves. You can get fancy and use a transit, or boards, strings and a level, whatever. Even looking at contour lines on a topo map is better than nothing. You say there isn't much head, but some people might say that about 6 inches and others about 6 feet. We don't know what you mean, and you may not, but that number is one of the factors in the equation for estimating how many watts you'll get.
The other main thing to know is the flow rate. Assuming this is too big to hold a bucket or barrel under, can you measure the flow with a weir? I'm lucky in that all the water through my brook runs under a bridge through a 5 or 6 foot diameter culvert, so I could walk down inside the culvert and hold a 5 gallon bucket out to catch the water. 2 seconds and it was full, so that's about 150 gallons/minute.
Another question is how able you are to build something like a dam or even a weir according to local regulations. Even if there are legal reasons you can't do that, there might still be options. Google search for "run of river" systems on this board. If the situation works out right you can put a propeller underwater or a paddle wheel with the paddle tips dipping in the water, even if you can't build anything permanent or block/divert the water. Start with this page for a small system example: http://www.otherpower.com/otherpower_hydro.html (not run of river but low head at least).
A picture might help us get a sense of what's there. Another thing would be to give us a feel for how fast it's moving. This could be 15 feet wide and moving so fast you can't walk through it, or it may be slow with frog's eggs and cobwebs. If you toss a stick or something in and time how long it takes to move a certain distance that might help. If this is a high-water time of year for you that's worth bearing in mind too. The rule of thumb is that you don't want to use more than half the water available, mostly to leave some for fish.
I bought a copy of this book and read it: http://www.amazon.com/Microhydro-Clean-Power-Scott-Davis/dp/0865714843/ref=pd_bbs_sr_1/102-3209038-3
143326?ie=UTF8&s=books&qid=1179265081&sr=8-1
I don't know what other people think about it, but I thought it was fairly useful. You might be able find or request something through a local library. There's also tons of stuff on the web if you've got online time to read it. There's a hydro section here but it's small. There's also a microhydro group in Yahoo groups. None of this technology has changed much in the last 50 years or so other than Neodymium magnets, so almost anything you can read about it is better than nothing. Avoid people trying to sell you things, at least at first.
Alan
