Yes, there's a canopy "tour" at Blue Creek near Punta Gorda in far southern Belize. It's operated by IZE. It was built by someone else -- can't recall who, maybe Raleigh International. You put on a harnass and walk along a platform high in the trees. It costs US$20.
More information is in my new Belize First Guide to Mainland Belize (will be available next week, 288 pages, 75 photos and maps, US$14.95 from Amazon.com and elsewhere, or included in a Belize First sub as a double issue).
Belize First Magazine http://www.turq.com/belizefirst/