I like using the web to save money. I use all of these sites to find the best deals.
istockphoto.com: A great source for really cheap stock photography. They have a huge selection of high resolution images sold for $1-$3. I've found this useful for all sorts of projects. Its an easy way to make your project or presentation look very professional.
addall.com: When purchasing a book online, you need to start here. It is a very comprehensive comparison engine that will find the best deal. It factors in shipping and can even search used books on half.com and amazon.com.
auctionsniper.com: Bid at the last second on your ebay auction and win. This allows you to specify a bid and have it placed with only a few Aseconds left of an ebay auction, leaving your competitors with no time to respond. Sniping is only effective because most ebay users do not fully understand how the ebay auction system works and thus underbid assuming they will have time to rebid should someone outbid them. Use 'bikeperson' as your reference when signing up and we both get free credits.
biddingfortravel.com: People who have recently bid and won hotels on priceline.com post their results on this forum. You can use this to pinpoint exactly what to bid and sometimes predict what hotel you will get. This site is essential for getting crazy deals on four star hotels.
housingmaps.com: a mash up between craigslist.com housing listings and google maps. This is very useful if you are looking for a place to live in a very specific neighborhood.