Seed exchange sites

A quick search in Google for seed exchange gives almost 10 million pages. I'm sure there's not that many exchanges so let's start at the top and see where the exchanges are and what are their terms of exchange:

The Garden Web forum gives users a list of many different seed and plant exchanges , I checked out the seed exchange. This forum allows growers with something to offer the chance of swapping seeds. What I don't know is if growers swap in pairs or if they can receive credits for providing seeds.

This exchange and others like it seems to be limited to the USA. I'm more interested in seed exchange in a tropical setting. I'll start with Uganda where I live...