I wish I had this resource when I was first learning algebraic number theory; I strongly recommend it to any student of algebra, number theory or cryptography, as well as people interested in special functions inside and outside of pure mathematics.
Apart from the specialist interest, I think the webpage design has a nice combination of form and functionality.
I've spent much of the last 7 years on work related to the LMFDB. It's very good at what it does, and is also a very good gathering place for computational arithmetic geometers, number theorists, and mathematicians.
I remember using this site for checking my work in a number fields course years ago; I never realised it had so much more