Some good first places to check for free movie links are WatchMovies, Alluc, TheOnlyDevice, and TVDuck. These sites are directories for other sites that host video. You can type in your movie search query, and a bunch of options will appear. Movie links periodically go dead or disappear, so searching an updated database first will save you time.
You can also find movies on Hulu, Megavideo, or Tudou (Chinese). These are sites that host videos. Sometimes if a video has just recently been added, it won't show up in a database search, so you can check these sites directly for the free movies you want to watch.
Many online movie videos work better when viewed in Firefox. If you choose to download it, get it directly from the Mozilla website.
Some warnings about various streaming players: The Youku player is very slow to load, so your best bet is to pause it for a few hours, and then watch it after you've exercised or done some housework. DivX players have good resolution, often DVD quality, but they are funny about fast forwarding and rewinding.
Free online movies on the Megavideo player have good quality, but there is a time limit to how much you can watch in one day. To get around this, you should pause the movie to let it fully load (10-20 minutes), and then disconnect from the internet and watch as normal.
Remember, it is perfectly legal to watch movies online if they have been released on video. However, downloading them is illegal, and in is not worth the risk to your computer should some sites have viruses.