If you gig with your guitar or rehearse, or even just use a strap at any time, it's worth investing in a set of strap locks. There's nothing worse than seeing your baby fall to the ground in front of your eyes...Nooooo!
Everyone knows about Schaller and Dunlop strap locks. They are the two big locking systems out there. Schallers can be great, but personally I've had them fail before.
Schaller strap pins sometimes get worn down over time and eventually they don't lock like they should. Other Schallers have literally exploded on me due to the spring they have. Other problems include the lock not staying tight on the strap. These babies will work, but they require maintenance.
I've used Dunlops less. When I did they seem like a good system. The strap peg is a little more obtrusive than the Schaller. The only knock I have on them is if they ever come apart on you, putting them back together requires skills akin to a mechanical engineer. Several washers and a C clamp washer that can seem impossible to re-attach. You just don't want to deal with this. But inevitably you'll need to take them apart for maintenance at some point, so be prepared.
Some new competitors have cropped up, such as Ernie Ball Super Locks and Hennessey strap locks. I've tried the Ernie's and they feel solid. As for price, at the time of this writing, Schaller and Dunlop locks set you back $14-$15. Ernie Ball is $18, while Hennesseys are an attractive $9.
Now that we've covered the standard metal locks, I'm going to suggest some less standard systems. (Please see the Guitar Strap Lock Hack at bottom.) Basically this involves buying a couple Grolsch beers and getting two rubber washers out of the deal, which you can slip over standard strap pins to keep your strap secure. To my mind, this is one of the best ways to go. While not a true "locking" system, they're cheap, easy to use, and you get a couple beers out of the deal!
While not really strap locks, another option are guitar straps that have locks built in. The two best may be the DiMarzio ClipLock and PlanetWaves locking strap, costing $24 and $13, respectively. I've used both and like both. The DiMarzio system is rock solid. Only downside is it requires you leaving a short nylon locking mechanism at each strap pin. They can get in the way inside a case and possibly scratch the guitar, but they are easy to clip and unclip, and you can add additional locking pieces to several guitars ($9/pair) so that one strap can lock with all your axes. The PlanetWaves strap is easy to use and once locked is rock solid. I've seen them rub some marks into the finish on some guitars over time though. Pick your poison--my favorites are the non-standard ones here at the end--but use something or pay the price. PS: My favorite desperation strap lock? Run to the nearest fridge and grab that C-shaped plastic "clip" off a loaf of bread. Work it over your strap pin and it'll work the same way the Grolsch washer does--admittedly without the same security. But hey, wadda ya want for free?!