.tk is the country code for Tokelau
http://www.iana.org/root-whois/tk.htm
If your website became successful, would you trust your domain to a country you can't find on a map?
While
http://ww.dot.tk is giving away free .tk domains, and that might be an incentive to those on a really tight budget, you should stick to .com. You don't run a business only thinking about saving a few pennies in the short-run --- long-term, you're always better off with .com.
If you were building a restaurant, would you build it 1000 miles from the nearest road, or in downtown NYC?