As of September 2012 ADA Fruit gets two gold stars. (The site is somehow related to "Lady Ada", so I use the pronouns "she" and "her"). First, she sells the "Beagle Bone black" (an ARM based credit card sized computer with ethernet running linux for $45!!). Also her website is the real deal, not the monte-carlo experience you get at Sparkfun. And there are some unique products that they design and manufacture. Key among these are the "Boarduino", which is an Arduino designed to plug into a standard breadboard (what a stroke of genius, too bad it doesn't use the 32u4 chip).
ADA Fruit and Sparkfun complement each other. ADA Fruit gets a big thumbs up for a superior website. Sparkfun has a lot more small items. If you are on the West Coast, be aware that Sparkfun ships out of Denver, whereas ADA Fruit is in New York City.
Sparkfun has lots of cool and interesting stuff, but their website frustrates me. Perhaps the greatest irritation is trying to use their navigation bars. Just use their search tool and be done with it. Also, when you find an item of interest, scroll to the bottom and look at the list of "what other people bought along with this". It is kind of like an Easter Egg hunt poking around at their site. You never know what you are going to find! I do give them points for having some unique products that they have designed and manufactured. They do have a lot more components and small parts than ADA Fruit.
Tom's Computer Info / tom@mmto.org