When you buy a new computer or TV from a big box store, they usually aren't making that much money off of that actual product. Where they do cash in, however, is by convincing you to buy tons of peripherals at incredibly high markups. (And don't even get me started on extended warranties--)
Next time you get pressured to buy extras like TV, S-video, audio, Ethernet, USB or firewire cables, tell them you're going to Monoprice.com, where you can get all of them for under $5 a pop. The site also has tons of converters, adapters, and even HDMI switches (Gizmodo) for a fraction of what you'd pay in a retail store. Monoprice also stocks cheap car charger accessories and USB hubs. The site doesn't explain much, though, so it's best to know what you're looking for going in.
(Image courtesy troutcolour)