Hey haven't seen these sites mentioned yet.

1) Dario Alpern's Generic Two integer variable equation solver for quadratic Diophantine equations in two variables, available in JavaScript and Java versions. He also has other tools on his site.

2) Google Scholar I use mainly for finding published papers, often there are downloadable .pdf files, good for people who don't have University access to JSTOR and such.

Hope these are useful to someone!