log4gm 
- provides console global object used for logging if its not available (it won't do a thing, then, though).
- binds console from unsafeWindow (e.g. firebug's console) to Greasemonkey's context
- provides a log4j-styled logging level differenciation (you can set log level and ignore messages below certain severity).

Please not that log4gm does not provide direct access to wrapped console object - only the limited set of methods listed below.

  /**
   * Sets log level property (default: INFO)
   * @param {String} level logging level: DEBUG / INFO / WARN / ERROR / LOG
   */
  setLoggingLevel
  
  /**
   * Always log into console
   * @param [...] Zero or more arguments.
   */
    log,
    
    /**
   * Log arguments into console unless severity is set to LOG (lowest)
   * @param [...] Zero or more arguments.
   */
    error,
    
    /**
   * Log arguments into console if severity is set to DEBUG (highest) or INFO (second-highest) or WARN
   * @param [...] Zero or more arguments.
   */
    warn,
    /**
   * Log arguments into console if severity is set to DEBUG (highest) or INFO (second-highest)
   * @param [...] Zero or more arguments.
   */
    info,
    /**
   * Log arguments into console if severity is set to DEBUG (highest)
   * @param [...] Zero or more arguments.
   */
    debug