Inkscape is free and open-source vector graphics software which runs on Windows, Mac OS X and GNU/Linux and uses the W3C open standard SVG as its native format. Inkscape can output as TikZ if you want to draw it first and then tinker more in TikZ by playing with the code.
The Ipe extensible drawing editor for creating figures in PDF format supports making small figures for inclusion into LaTeX-documents as well as making multi-page PDF presentations.
TikZ: A simple visual editor for creating commutative diagrams. Also see link to github for TikZEdt standalone software for producing figures with a built-in graphical editor and a code box.
Wolfram Alpha is a set of data and algorithms used for answering questions in a variety of subject domains. The aim of this product is to enable users to quickly engage in data computation and information visualization.
Arxiv. Fields: Physics, mathematics, computer science, quantitative biology, and statistics.
Open access to research papers in physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and systems science, and economics. Consists of electronic preprints (known as e-prints) approved for publication after moderation.
Project Euclid is a not-for-profit online publishing service that provides access to journals, monographs, and conference proceedings in the fields of theoretical and applied mathematics and statistics. It is jointly managed by Cornell University Library and Duke University Press.
JSTOR's Mathematics & Statistics Collections unites 319 books and journal titles in the mathematical and statistical sciences from existing JSTOR collections. Journals in these collections overlap with the Arts & Sciences I and II collections, as well as with the Business and General Science collections.