I have tried the aforementioned plant, bottles of water, old CDs dangling on strings (garden art, v. pretty), orange and lemon peel, moth-balls in little bags made of old tights (actually they were new tights, I rarely wear tights so had to buy some specially) on garden canes, Jeyes fluid in yoghurt pots buried in the lawn, tea bags soaked in Olbas oil, pepper, paprika, curry powder, proprietary deterrant gels etc.
I did buy lion poo pellets but didn't use them as the instructions said it can make the problem worse before it makes it better (cats see it as a challenge) and I couldn't face it.
Also haven't tried the sonic sound repeller things as they are expensive and might not work.
I also keep a stash of corks by the back door which I chuck at any cats in my garden (as yet, I have never managed to stopper an arse - that is my ultimate goal. In fact, I've never managed to hit a cat...)