The first I heard of it was when I was watching a gardening show on TV called Vasili’s Garden . The lady he was interviewing about her garden told him that she used aspirin on her tomato plants. So last year I used it, and not only on my tomato plants, but all the other vegetables I was growing. I used 2 tablets in a 9 litre watering can every 3 to 4 weeks. All the vegetables were all great but in saying that, it may have been just a good year for growing. I told 4 of my friends who also grow vegetables and all had the same result.
Plants make salicylic acid, the active ingredient in aspirin, to trigger natural defences against bacteria, fungi and viruses. Thus, Aspirin is an activator of "systemic acquired resistance" (SAR) However, plants often do not produce salicylic acid quickly enough when attacked by a microbe to prevent injury. Spraying aspirin on plants speeds up the SAR response.
http://ww20.sbs.com.au/vasilisgarden/index.php?pid=gardens&active_anchor=683
http://www.ehow.com/how_2126099_use-aspirin-grow-tomatoes.html