It depends on what you want to photograph and what type of trip you are taking.
I'll generally assume you want to photograph wildlife. Insects are certainly the most common and there are many unique ones there. At minimum you will need a 100mm macro to photograph them. If you're serious about insects, an MP-E 65 is also a good route. Note that realistically you will only have time to photograph insects around your camp (but they'll still be abundant and easy to find). When I went there the tour guide and others in the group were not willing to wait for the lone insect photographer. Better yet if you want to photograph insects take a private tour.
You'll also see a lot of birds, monkeys, peccaries, agoutis, and other critters. When I went to Manu, Peru I also saw giant otters, caiman (black and white), a tapir, and a jaguar. What type of lens you need depends on how you will be seeing them.
- By far the most effective way to see wildlife is by river. In Manu we travelled by motorized canoe for long periods of time and sat on a raft in an oxbow lake. The oxbow lake was absolutely incredible and every way you turned there was something interesting. A 100-400 would work very well there because some animals came very close (giant otters) while others were further away (monkeys).
- When travelling by canoe, basically the longer the telephoto the better. Many subjects were quite far away - though there were a few exceptions (we came close to a family of capybara). When I go back some day I would certainly want a 500/600 lens + extenders or at least the new 200-400/1.4x.
- At other times you will be in one place as the animals will come to you. We used this to view the macaw lek, cock-of-the-rock, and the tapir. For the macaws and cock-of-the-rock a 400mm lens would still be very useful as they were still quite far away. A zoom lens may give more flexibility. The tapir came quite close and could be reached with a normal lens - but it came at night so you'll need a camera that can handle high ISO and a fast lens.
- When walking in the jungle, you can generally forget about getting a good shot off. Most wildlife tends not to stick around very long when spotted so you're not likely to have the chance. The exception are some of the monkeys - and you'll need a long and reasonably fast lens to reach them. Be advised that if you approach too closely they may urinate or throw things at you. I did have more success walking alone on the paths around our camp - where I had a lengthy encounter with an agouti and several wooly monkeys and very brief encounters with a snake and a jaguar. To be honest I would spend more time photographing the insects and the trees/plants as few critters will give you the chance.
- Sunsets on the river and mist. Bring a decent WA lens for these situations as they can be absolutely magical. This is a shot I took when I went - before I got into photography at all or had an SLR.
http://www.flickr.com/photos/kirispupis/93468554/in/set-72057594067259015/Weather sealing is a concern. Remember that they call it the rainforest for a reason. Regardless whether your equipment is weather sealed I would bring rain covers for you camera + lenses. Also note that if you cannot afford some of the larger lenses, renting is still an option.
Note that this advice applies mainly to neotropical jungles. If you are travelling to more open areas in SA - notably Los Llanos or the Pantanal - they aren't as applicable.
