There are a lot of sites that allow you to watch free content ( Like Movie2k, fflims.org and etc) but nothings free so what am I giving up? Does connecting to their site give them free access to my computer?
Stay away from questionable sites. Try http://archive.org, which is great for both entertainment and research, or choose a free, legal site like http://essimi.com.
Provided the website is somewhat 'reputable' or rather well-known then it's unlikely to give you viruses or something. Google information about a site if you're in doubt. Make sure you have some anti-virus software installed even if it's just AVG Free as you can never be 100% safe from viruses even if you think the site sounds okay.
Remember that the quality of free movies on these sites are not always great so you may find that in order to enjoy the film more, you may have to go elsewhere.
If the site wants you to download its plugin, player or whatever addon, it's suspicious. Maybe the whatever you are downloading maybe malicious or a trojan!
There is also a somewhat new breed of viruses that can be inserted into media files. They've been around in various forms for about a year. Saving or streaming these media files to your computer may result in a nasty infection. I've noticed that Avira and Sophos have only recently updated their virus definitions to catch these ones. So, Surf Safe!
As you say nothing is free but with an exception!
Malware, using the wrong sites can you loads of malware all for free. Do yourself a favour and stick to legitimate sites before you are back here with questions like "How do I remove this and How do I remove that"
The only real major concern is if the content being viewed is pirated (illegal copies and shared illegally) or if it is legitimately free. I'm sure they, like most websites, track some of your information through cookies, but mostly they are not looking directly into your computer (such as can be done with torrents).
Both of the sites you mentioned are streaming copyrighted content, so the first thing to consider if whether you want to head down that route. They're definitely breaking the law, and by watching the films, you may be too.
Assuming you ignore that issue then most of these sites are loaded with pop-ups and potential malware, so you visit them at your own risk. If you can find a legal alternative then do so.
Streaming video sites do not steel your computer unless the particulare site is not clean one, but video providers can collect information about users to check video usage some do allow users to inspect the information.
Video Protection, Security & Privacy