It's a carpooling site. I use it to travel between Krems and Vienna for 4-8 Euro each time. The train costs 15.40 Euro one-way.
A friend was supposed to travel to Prague from Vienna with one guy but the man didn't show. I tried to communicate with another driver in German but we didn't understand each other well so he turned up one day when I didn't need to go to Vienna. So. There's that little drawback. It's mostly good though.
Please PM me if you know of more or better options. I feel like I'm advertising for blablacar if I show just one option and I don't mean to.
UPDATE 29 July 2016: If you're in Denmark, use gomore.