Anyone know of any reliable (and preferably cheap) companies for wildlife conservation volunteer work abroad?

I'm looking to do some volunteer work this year involving conservation. I have of course Googled this and have a few ideas of my own, but was wondering if any of you guys have any personal experiences with companies you'd like to share?

What I'm looking for really is something which is fairly reasonably priced. I know this is not going to be cheap - but the lower the cost of living, the longer I can stay! I'm looking to spend anything between 4-12 weeks out there. As far as location is concerned, I'm really not that fussy at all! I just want to go somewhere beautiful surrounded by nature that I can not only work with, but photograph and film. I have a couple of places in mind, at the moment I fancy Brazil, South Africa or China. I can only speak English and Spanish (rather poorly!), so a primarilly English speaking programme would be needed (from what I've read, this seems to be the case for many).

Any help with this would be really greatly appreciated. I'm really keen on making this happen but need a bit of guidance! :)