The original Gunson's Flashtest, adjustable to check the strength of the spark, you might pick one up on eBay cheaper than the Laser 5655:

A basic go/no-go but good value at around £8: (Sealey VS526)

Also no go, but check all four plugs at once at a price - £22: (Laser 2780)

Adjustable to check spark strength - expensive at £30: (Laser 5655)

Another 'go/no-go' one but only needs to be touched onto the outside of the lead so easy to move from one lead to the next: