Which channel lets you watch movies and tv serials for free on ROKU?
hm9m, Gilgamesh's suggestions are a great place to look, especially Crackle.
popcornflix: i'm hoping they will get their bugs worked out because it looks like another good place for bunches of movies.
dreamtv: has some free and some premium [paid] content that you might want to explore.
koldcast: has a lot of fun, free content
nowhereTV: includes most of what's on the internet archive as well as news, PBS, some HGTV and FOOD network [there are also separate PBS, HGTV and FOOD network channels] some Lifetime shows, adult swim and a whole lot more
retrovision: classic movies [frankly, there's a lot of overlap on several channels with public domain content]
and bunches more.
You can skim the list at http://www.catastrophegirl.com/rokuchannellist.htm
for a (nearly) full list of channels available, both public and private. There, you can find out about channels such as HGTV and PBS for content that you would expect from other sources.
cbojar, you said "nearly" full list... if you know of channels i need to add to the list, please fill me in. i want it to be as complete as possible. i'd love it if people would let me know when they want me to include their channels and list their developer credit.