Go to a cash back website like Topcashback.co.uk and you'll see a list under the insurance section. I used insurance4carhire and got £10 off a £40 annual UK and European policy. USA/Canada policies are £110 at the moment.

What I liked about these policies is that they cover for roof, windscreen and hub cap damage which at one point even the car hire companies expensive policies didn't cover - but I have noticed some do now.

One point though is that some car hire companies will then block your card for the excess amount. In Australia that was $3000. Luckily for me I have many cards and gave them one I wasn't planning on using, but if you only have one card with a low limit you may find yourself in trouble.