aaLogWP

Allows developers to log variables, see the call stack, benchmark code; all to a logfile, so your page doesn’t get polluted with log messages. Each log message records the date and time, IP address, requested page, and file and line the log message came from, as well as the information to be logged.

The plugin has an options page which allows you to randomize the cookie and logfile, for security purposes; as well as turning logging on and off for a given user. (Via cookie.)

Documentation for the aaLog class can be found at the NotoriousWebmaster blog.

Documentation for the aaLogWP plugin, which includes the aaLog class, can also be found at NotoriousWebmaster blog, though on a different page.