YouTube's coming attractions now include movie rentals. The Internet's most popular video channel will make its debut as a rental outlet Friday to help promote some of the movies that will be shown at the upcoming Sundance Film Festival in Park City, Utah. It's part of a test that YouTube hopes will encourage more studios to rent movies through its site, eventually creating a new financial stream to supplement the Internet ads that bring in most of its revenue. The first five films available to rent through YouTube will cost $3.99 for a 48-hour viewing period. Movie studios will be able to set their own prices, with rental viewing windows ranging from one to 90 days. YouTube will get an unspecified commission from each rental. The expansion announced Wednesday marks the latest step in YouTube's evolution from a quirky and sometimes edgy Web site that showed free clips posted by wannabe stars, showoffs, bored teens, lonely hearts and video pirates.