Writing prompts are any words, images, phrases, or ideas that stimulate your writing. One great source is the "Lovestruck" type column in which people try to establish contact with someone they saw somewhere but didn't have a chance to ask out, or were to shy to ask for a date. These are especially good if you want to write a romantic comedy. Here's an example from a recent issue of thelondonpaper:
"Rugby-player-build guy. Got on the Piccadilly line with me on 10 September, 5.30pm, at King's Cross, where I almost sat on your lap. Both got off at Covent Garden. You went for a drink with a mate in Wellington Street. I went to work at the theatre. You were gorgeous. Amy." (Hmm, kids, can you spell 'stalker'?)
If a paper in your area has a column like this, keep an eye on it--it might just spark an interesting story idea.