Preschoolers like to open things -- cabinet doors, toy chests, you name it. There are lots of places you don't want your child to access, and putting a lock on the door might be the way to keep him out.
Before slapping locks on all your cabinets, which can be costly, start by moving cleaning products, medications and other items you don't want toddlers to get into to higher cabinets out of kids' reach.
Never put a lock on a refrigerator, freezer, clothes dryer or cooler. These have limited air, and kids must be able to open them from the inside should they fancy them desirable hiding places. (Don't put it past them.)

Image courtesy of PricegrabberThese cabinet locks hide inside your cabinets, perfect for unseen protection. They use a key placed on the outside of the cabinet to unlock. As long as you leave the key in place, the cabinet stays unlocked. This is really good for Grandma's house -- these locks can be disabled when the wee ones aren't around.
These are additional locks for the starter set above (includes four locks without a key). Remember to move all dangerous items to higher ground so you don't have to buy as many locks. Put pots and pans in lower cabinets so kids can get them out and bang on them with wooden spoons. There's no safety reason to do this -- it's just fun!
This lock is easy to open with one hand and doesn't require any hardware to install. It is perfect for mirrored cabinets. Once installed, though, this lock will keep on locking -- there's no way to disable it until needed. The adhesive mounts won't damage cabinets, so you can take them off easily when the kids go to college.

Image courtesy of PricegrabberAnother magnetic lock, these also hide away unseen. Magnetic locks are nifty, but losing the key is a real possibility. Kids are designed to learn, so there's a chance Junior's going to figure this key thing out pretty easily. Don't let the little ones see the magic.
Designed for swivel cabinets and drawers, rather than doors, these locks can be opened with one hand. Installation requires securing with screws into your wood cabinets. These may be more permanent than you were expecting, though they are useful.

Image courtesy of PricegrabberThese locks slide over cabinet handles and lock in place. These are quick and easy for Grandma's house, where Junior might not always lurk. Unfortunately, enterprising youngsters seem to take these obvious locks as a challenge, not giving up until they've managed to get the lock off or destroy the handles.

Image courtesy of PricegrabberThese come in handy for keeping kids out of rooms. Put them on regular doors and flip closed when you want your little princess to stay out. When the kids are gone, just flip them back.

Image courtesy of PricegrabberIf you don't want the wee ones to splash around in the wee pool, these locks will keep the lid on. Put them on all the toilets in the house to keep kids out of the bowls.

Image courtesy of PricegrabberKeep kids from turning on the stove with these stove knob locks. They slide behind stove knobs and keep aspiring chefs from turning on the heat. These are an especially good idea for gas stoves, reducing the possibility of
carbon monoxide poisoning.