Registrations used to be open but now they require a Japanese mobile number. There is also no English version of the site, so it can be quite intimidating, but there are lots of people on there who want to learn English or just want to learn about foreign cultures. At first you will learn a few words, bits about Japanese culture, and have your bad habits in Japanese pointed out for you, but after a while you will find yourself speaking fluent Japanese.
Facebook and Skype also have many Japanese people on there looking for cultural+language exchange/friends, but mixi seems to have a lot more.