Lots of things sprout quite spontaneously and undesirably in the larder at this time of year, and can be planted as an alternative to throwing them away: carrot tops, potatoes, onions and garlic, for example. For every clove of garlic that you plant, you get back a whole head in the autumn when the foliage dies down (if you're lucky -- they'll be smaller than the commercially-grown ones, though).

Tomato and squash seeds sprout on the compost heap if given half a chance, and flourish mightily, though they never seem to fruit. You can plant the seeds from chillis and grow them very successfully on the windowsill -- it doesn't matter if the individual fruits are small as you only need one per dish anyway.

You can also germinate lemon seeds and grow them on for a few years, though the chances of getting a lemon tree out of it at the end are tiny...