Peterbilt/Kenworth has ECAT, but you need a dealer to sponsor you. I think they charge $300/year, some dealers wave that charge if you're a good customer. We don't pay because we have 23 Petes and buy a lot of parts.
I know international and Volvo have the same thing, but I do not know cost and if it's available to small customers. I haven't used it in years, we got rid of all of our Volvo's, but I remeber it NOT being very user friendly.
Access Freightliner is free for O/O's and small fleets. Cummins QuickServe is also free for up to 5 engine serial #'s.
I don't know of anything from Cat cheap or free unless you're a huge customer, but Cat SIS is included with our ECAT for parts look up only, no access to service info.