During work on my Master Thesis here in Austria I came across two very useful websites.
The first one provides a very fast online plotting tool for 2D and 3D functions that allows the user to specify basic viewing options. The resulting images can be saved in various formats. Another very nice feature is the permalink option which basically allows users to share their plots with others.
The second website is called Googlefight. As the name suggests, the idea behind it is fairly simple: search for two given terms and see which one of them is more popular in the eyes of the internet's all-mighty (i.e. which term yields more search results in Google). I found it very useful when I was not sure which term is more widely used in a given context.