There are a few roku channels that provide free content, like Crackle. They generally have commercials but they don’t run for several minutes straight. Some, like A&E and PBS, have limited free content.... maybe a few episodes or seasons of their programming.
Best bet is to pick up a service like Netflix and/or Hulu.
Also, look at getting a HD digital antenna for network programming. We get quite a few channels with ours but mostly just watch local news and weekend sports.
Some network websites used to have the last few episodes available for free, I don’t know if they still do that.
CBS used to have all Star Trek TOS online for free, now they say that 16 eps are free....
http://www.cbs.com/shows/star_trek/