There are quite a few places I troll to find free (and good) resources. One of them is monergism.com, a great reformed site (and don't forget the bookstore). They have a bunch of amazing theology resources, including an extensive mp3 archive full of some incredible teaching. If you are going on a long car ride, I heartily suggest downloading a longer-than-necessary playlist and let the miles just fly by.
They also have a bunch of free ebooks in both .mobi and .epub formats. Authors range from Augustine to Van Til. Their bookstore has more ebooks, the most beneficial of which are the massive omnibus editions that compile everything you ought to know about ________. These can range from heady topics like Justification to more general things like Calvinism. Because there are typically about 8-12 full length books in each omnibus, they do charge a bit for them, but to have all of those resources together is worth a few bucks.