Flora-2, aka Ergo Lite, is an open-source subset of Ergo, Coherent Knowledge's knowledge representation/logic programming system. It implements only some of Rulelog and doesn't have an IDE.
Flora-2 runs on top of XSB.
ErgoAI implements more of Rulelog and does have an IDE
There's also Sunflower, an IDE from SRI. Life signs minimal.
cf tutorials at Tutorial on Flora-2, Sunflower User Manual section on Flora
Etymology: "F-LOgic tRAnslator"