Keyboard symbols charts are available at Alt-Codes.net and Nouilles.info, as of 2015. Both websites feature charts that provide keyboard shortcuts and instructions for using special codes.
A keyboard symbols chart is available on the home page of Alt-Codes.net. Clicking How to Use Alt Code Characters displays instructions on how to insert characters.
Symbol charts are available at Nouilles.info by clicking Charmap at the top of the home page. The chart contains keyboard shortcuts for Mac and Windows, HTML code, and a description of each symbol. Links for support documents for inserting special characters on Mac and Windows computers are also available on the page.