MochiKit has the following functions (included here with full namespace resolution):
MochiKit.Logging.logDebug() // prefaces value with "DEBUG: "
MochiKit.Logging.log() // prefaces value with "INFO: "
MochiKit.Logging.logError() // prefaces value with "ERROR: "
MochiKit.Logging.logFatal() // prefaces value with "FATAL: "
MochiKit.Logging.logWarning() // prefaces value with "WARNING: "
There is a lot more to the MochiKit.Logging namespace than this, but these are the basics.