Iron Borders: This is one of the easiest borders to install. There's no digging involved, you just shove the prongs right into the ground. Use where you don't have to worry about turf creeping into a bed.
Terra Cotta Pots: These are a quirkier option. You can either turn them upside down or plant a single plant in each and line them up. Some people like to take this whimsical look even one step further and use abalone shells or recycled items. These borders will certainly add character. Use where you don't need to worry about turf creeping into a bed.
Plastic Edging: This is a nearly invisible border. It sinks almost all the way into the ground. This is more practical than it is decorative. It's usually used to separate a lawn area from a mulched planter bed and will prevent grass creep.
Garden Wall Blocks: These landscape blocks are very cost effective. They look like stone but are actually made of concrete.
Stone: You have a variety of colors and sizes to choose from. You don't necessarily have to mortar them together because some shapes stack well.
Bricks: If you're not a bricklayer you can still use these without mortaring them together. You can lay them vertically at a slant to create a formal looking border. Recycled bricks are sometimes available as well and they're much cheaper.