Sonic Cat Repellers
Pest Bye 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.
Selections Cat Repeller
Again works on a passive infrared sensor, I'm not as keen on this one as it isn't as easily adapted but it is cheaper *slightly* and it does come with batteries.
Defenders Cat Repeller
Probably the best known on the market. It is a motion activated Mega Sonic Cat Repeller. Green - so blends in (ish) to your garden and deters the heck out of cats without harming them. Hands up, I don't have this one - I prefer the Pest Bye but I have a friend in a gardening club that has one and he swears by it.
Rosewoods Cat Spray
This is a quality, fit for purpose cat and dog spray. It is made for use in the garden. You simply identify the areas that the cats are doing their business and spray around it. Obviously try and spray it on hard surfaces like fences or walls, rather than ruin your garden. It is noxious! Always wash your hands after using it.
I'm not a big fan of cat sprays - especially when they come in small 60ml sizes like this but it is on of the leading brands. I do however suspect this is more for cat owners to use inside to train their cat not to mess up areas of the house than for practical use in the garden
"Get Off" Scatter Crystals
I haven't used this one - mostly because through my own hard work the cats hardly come in any more - but I like the idea and it is made by Rosewood, same as the good old handy spray.
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