Internet Archive->Wayback Machine

http://web.archive.org/

I use this website all of the time; in fact, I use it to link to an archived page of the multiVAX website:

http://www.multivax.com/last_question.html

This site is most likely my favorite. This extensive archive of even the most obscure resources has provided me with finding many pages and websites that are no longer active.

Google

http://www.google.com/

I use Google as my primary search engine. It is immensely helpful for searching given their strict search operators, like intext: and site:.

Their image search tools are also very helpful.

Wikipedia

http://www.wikipedia.org/

This free encyclopedia is another website of utmost importance for me. When it comes to learning about various topics, Wikipedia is truly indespensible.

Zamzar

http://www.zamzar.com/

This list would truly be incomplete if I were to exclude Zamzar. This service, which performs multimedia conversion between many file formats, has been an extremely beneficial tool.

I have used this for various other conversions which I could not complete with GIMP, VLC, or another software.

latex2png

http://www.latex2png.com/

This website has been very valuable to me, by allowing me to create beautiful, high-resolution formulas, which I have used on this website and often in my academic work.

ChemDoodle->2D Sketcher

http://web.chemdoodle.com/demos/sketcher/

In a similar vein to latex2png, I use ChemDoodle's 2D Sketcher for much of my academic work. It is a great tool for creating 2-dimensional chemical structures online.

Sci-Hub

http://www.sci-hub.[..]/

Enter a paper DOI, and never get stuck at a publisher paywall again.

At the time of writing, sci-hub.tw is the active domain name.

VPN Gate

http://www.vpngate.net/

I use the .ovpn files offered on the site in conjunction with Tunnelblick, an open-source VPN software, in order to quickly connect to a VPN.

It is very easy to connect to a VPN, and the academic basis for this hub creates a minimal VPN solution without the flashy, ad-filled experience of a typical VPN consumer product.

IETF Tools->Requests for Comments

http://tools.ietf.org/rfc/

I use this very often for consulting RFC documentation. A particular favorite of mine is RFC 1149.

Wolfram|Alpha

http://www.wolframalpha.com/

This is the best online calculator I have found.