I wouldn't say it's nice but moodle is an option... haven't used it in several years so I'm not sure as to the current state of the project...
Doing this as a startup as opposed to company internal training or an educational institution means that you want to present yourself well and create a user experience that you're proud to send your customers to. Seems hard to find!
There has been support for add-on modules for a while now, but for the hackers, the 3.7 release later this year is a fairly big architecture change and opens up the entire platform similar to how Wordpress manages their plugins in terms of hooking, etc.
Let me know if you have any questions and I'll do what I can to assist.