Pestbye Cat Repeller
Without doubt the "Daddy" of Cat repellers. You can mount it on a wall or stake it in your garden. It works on a motion sensor and you can decide how sensitive you want that sensor to be. Battery operated and relatively inexpensive.
Stay Away Cat Scarers and Sprayer
This is incredible. Combining my two favorite ways to get cats out your garden, you can't do better than this Stay Away product. It combines the infrared sensors of the normal cat repeller and uses a spray technique to scare the cats away. AND you can either load it with water, so it genuinely sprays OR just have the sound.
Strobe Ultrasonic Cat Repeller
Another great product from Pestbye. This is a solar powered version of the Cat Repeller and "strobes" ultrasonic sound at the cats to scare them away. Again, wall mountable so if you have a garden that does get the sun then perhaps this one is for you.
Havahart "Critter Ridder" Spray
This is a quality spray for cats and other "critters" - which could be any mammal really. Always worth keeping that in mind if you have dogs or the like - cat proofing your garden will likely dog proof it too!
Dog and Cat Be Gone
Great quality spray, slightly bigger brand - if you are into that kind of thing. : ) It tis safe for humans and is relatively rain resistant - which is always a plus. Half the battle with sprays is keeping them on the area long enough.
Defender Cat Repeller Crystal Gel
Essentially these are scatter crystals which you can put on your lawn, on stone paths etc that are all embalmed with the nasty chemical that cats don't like. As well as preventing cats doing their business, it does stop them digging too.