Basil: Ants in the kitchen drove me mad last summer – until I discovered that basil is Ant Kryptonite. Buy a pot from the supermarket for £1.99 or thereabouts, put it in a bigger pot and stick it on the window sill. Water sparingly and replace every few months. It keeps the ants away – and it smells nicer than Raid.
Lavender: It grows in your garden (or your neighbour’s garden) for free – and flies hate it. Little bunches, hanging in windows or left on top of tall furniture, work well. Replace when the lavender smells of nothing but twigs. It seems to me that “true” lavender is much more effective that lavendula (the “cheaper” stuff, which has squatter, slightly pinker blossoms) – but this could be mere horticultural snobbery on my part.