Are there any congruence subgroups other than $SL_2(\mathbb Z)$ which have exactly 1 cusp? By congruence subgroup, I mean a subgroup of $SL_2(\mathbb Z)$ containing $\Gamma(N)$ for some $N$.

This question was motivated by the comments on the answer to http://mathoverflow.net/questions/66819/generators-of-the-graded-ring-of-modular-forms/211351#211351. In particular, if there are no such subgroups, then I believe all congruence subgroups other than $SL_2(\mathbb Z)$ are generated in degree at most 5, with relations in degree at most 10.

I have a feeling the answer to this question may be well known, but I could not find it in Diamond and Shurman or by a Google search.