Links to modular forms related pages

J.E. Cremona's Elliptic Curve Data Huge collection of Elliptic curves data.
John Cremona's book Algorithms for Modular Elliptic Curves Online version of the book
W. A. Stein's Modular Forms Database Very comprehensive tables of elliptic modular forms and derived quantities

Links to further collection of links

John Cremona's home page Elliptic curves database, various further links to related sites.