My bike, a BMW R1100RT, came with a helmet lock.  I'll pause a moment while those of you familiar with the bike stop laughing.

…….

OK, got it out of your system?  BMW considers a tab of metal under the passenger's seat to be a "Helmet Lock".  Not real convenient, you have to unlock the seat under the taillight, try to fit a D-ring on the tab without dropping the helmet or the seat, and then get the seat fastened with that third hand the design engineer must have had.  Oh yeah, forget about it if you happen to want to keep the hard-luggage on the bike - it can't be done.

The Helmet Guardian does a great job of keeping two helmets secured while you're off the bike.  It has two keyed-alike locks, and simply fastens in place behind your license plate (using included security hardware, of course).  It's simple, effective, and oh-so-handy!