If I'm not mistaken, I've introduced norcalfella, futurenow, and DougieFresh (not sure he's a member of Small.to, but he's married to Jill) to each other.  And their common frugality led to a passed-around spreadsheet identifying the least expensive place to purchase a variety of items.

(e.g., I think paper products were cheapest at Costco, disposable diapers were cheapest at Wal-Mart, Albertson's had the least expensive fresh fruit...)

But when is the best time to buy which things?

This chart at Lifehacker purports to answer the question of timing.  Most of these make perfect sense, but I disagree (obviously) with the pricing on electronics and TV's.  In general, the best deals on those are either just prior to Christmas, or just before a key new release.  (For example, buying an LED-lit LCD HDTV with local-dimming, right now, is a great opportunity, because all the major manufacturers are introducing new models with gee-whiz Internetty features that proably aren't that valuable---so the best technology is being dumped from inventory to make space for the newer models.)