Centipedes. These are absolutely good guys -- they're carnivores: fave meals include slugs and slug eggs. ;D Millipedes, on the other hand, are vegetarian. ID to tell 'em apart: centipedes run faster than millipedes and have only one pair of legs per body segment.
Woodlice. Normally they're detrivores, eating dead plant matter and soft, rotting wood, so in the garden or on the allotment they're pretty harmless. But they can munch through seedlings, so if you have a greenhouse containing both big colonies of woodlice and lots of vulnerable plantlets, you may not be able to afford a 'live-and-let-live' approach.