Pro Publica has a new Congress API, which tracks roll-call vote data, member date (eg votes and co-sponsorship of bills), bill information for all bills since 1995, and supplemental data like votes by state and by party. Updates at least once a day.
api/propublica-congress-api is where you apply for an API key. You have to say what you want the information for (student or personal project, non-commercial, commercial). It's not really clear if it's free, maybe it depends on what you're using it for?