www.act.org - Official ACT web site. You can get a lot of credible and authentic information about the test here.
www.actstudent.org - Good web site for test takers. Lots of information on test registration, practice tests, college planning, financial aid. etc.
www.act-sat-prep.com/ - Another site that gives you plenty of information on the ACT. It contains tips for taking the test along with test information