Many websites make use of access keys to help power users do things quicker and reduce the amount of back-and-forth between keyboard and mouse. These shortcuts are different from site to site, and it can take a while to remember them. Additionally, websites don't usually go out of the way to advertise their use of access keys.

Enter this stylesheet, which places a high-contrast box next to page elements that specify access keys.

More information:
http://en.wikipedia.org/wiki/Access_keys
http://www.opera.com/support/tutorials/nomouse/#access
http://www.opera.com/support/tutorials/nomouse/

See this style in action before installing it!
http://danrasmussen.org/dev/?style=grayscale&accesskeys=y
http://danrasmussen.org/dev/?style=blorange&accesskeys=y

Requests and suggestions welcome at http://danrasmussen.org/contact