There are several open source and free radius server implementations, that I have come across in the past, but never actually used then so can't say which is the best/etc but a quick google search turns up a couple and some how tos to install, etc
The only implementation I have actually been a part of used MS Active directory to authenticate based on MAC addresses, but you probably don't want that.
By the sounds of it you want to set up a hot spot, which there are many many ways of doing. Again there are many open source and free implementations, eg Chillispot
But I agree maybe a review of these would be nice to save us commoners having to try them all
For certain you have to be lost to find the places that can't be found. Elseways, everyone would know where it was