The RetroUSB kits are pretty good. Just be aware that they only support gamepads..... multitaps, oddball racing controllers...ect not supported.
If you wander around this forum you'll stumble across my stalled "uni-con" adaptor I was going to produce. I ran into a wall during the test phase. The physical connectors for the various gamepads are hard to come by and when you do find them they can be quite expensive. If someone could find me a ready-made supply of connectors for around a buck a pop I would get back on that project and churn out a device that would work for just about anything.