How to simulate a low bandwidth connection for testing web sites and applications

Facebook just announced an internal initiative called "2G Tuesdays" and I think it's brilliant. It's a clear and concrete way to remind folks with fast internet (who likely have always had fast internet) that not everyone has unlimited bandwidth or a fast and reliable pipe. Did you know Facebook even has a tiny app called "Facebook Lite" that is just 1Mb and has good support for slower developing networks?