A cloche, French for "bell", is a miniature greenhouse that can be placed directly over plants.
A DIY cloche can be made from a plastic bottle; alternatively, just the bottom can be cut off and the top pressed into the ground, providing warmth and protection for seedlings. In cold climates this allows seeds to be planted a few weeks earlier. The cap may be used for further control - remove it in warm weather, replace it at nights. A simpler way is to leave it off entirely - this will still offer some protection (though not as much) while needing less attention (and less concern about letting the seedlings overheat by leaving the cap on during a warm day). Further info: An internet search for plastic-bottle miniature-greenhouse yields many relevant hits.