What is a good way to cat-proof (or child-proof) kitchen cabinets?
There are inexpensive child safety latches that can be placed out of sight in the interior of your cabinet. They are also easy to install and only leave a few small screw holes when removed. Some may use adhesive instead of screws, but they might be less dependable.

Such a latch consists of a bendable plastic rod with a blunt hook on one side, and is situated on the inside of the drawer or cabinet. The hook catches on part of the drawer or door and prevents opening unless the rod is bent downward simultaneously to disengage the hook.

These latches successfully prevent cats as well as children from opening cabinets more than one or two inches.

Links

For pictures and advice: Picture / Video / Pictures of installation

Some alternatives are discussed here.