Compiere was the first that I know of, and Apache has Open for Business. Both have spawned many others, and many have popped up without those roots. Do any of them have what it takes for a small business to operate out of the box or with minor configuration work? Which call themselves open with a crippled community option but charge for proprietary versions necessary to be useful? Which have significant ongoing development efforts? How many production users do they know about? Add those to the usual information about commercial products.