The register and login links shouldn't be visible on every page. Instead ask users to login when they actually need to log in (e.g. creating sheets). Contact us shouldn't be a big tab, instead something in the footer.
Your "Follow Us" "On Twitter" part should be all one line, and perhaps be reworded to "You should follow us on twitter <a href="bork">here</a> (see http://dustincurtis.com/you_should_follow_me_on_twitter.html)
The 1px border at the bottom of the page shouldn't extend past the footer, this looks pretty ugly.
The front page "Worksheets available" could be reworked into a "Popular Worksheets" ranking sheets on the number of views.
The text fields in the registration page should all line up, it looks pretty bad how they are at the moment. Also on that page:
"We'll only use your email to send you signup instructions" could be reworded to "We'll only use your email for signup", less words means people are more likely to read it.