They are around since 2003 and have a vast amount of users.
99 nice challenges (by now) await you on this well known english site, and they are adding more.
Many thanks go out to their staff, especially to comperr and xelix for their initiatives.
HackThisSite also helped to increase privacy on wechall and its affiliated networks.
What can i tell more than "Happy Challenging !"
Edit: to link accounts you need to enable this on your HackThisSite account:
http://www.hackthissite.org/pages/user/affiliate.php