It was menswear magnate Sy Sims who extolled "An educated consumer is our best customer." In the small businesses that are dominating the Maker Movement, many of them are taking this phrase to heart.
The poster child for informing customers is Adafruit. First with their tutorials and now with their new learn.adafruit.com portal, their tutorials not only on parts usage but actually how to make projects that may be the building blocks to your own projects is impressive. Grade A, gold star in my book.
Make magazine and their Maker Shed store have been in the game too although I have never thought they provided the comprehensive information available on Adafruit and some non-retail sites. Reading Make, the projects are way cool, but there is no specificity on trying to recreate the work yourself. More like Popular Mechanics than Popular Electronics.
Finally there is Sparkfun. My take is they focused on having a broad array of products, many of which one could not find elsewhere (as they took useful parts and placed them on custom PCBs to allow the average person to hook up wires and piece together projects). But their datasheets and other material often have not been comprehensive, leaving the person to figure out the particulars themselves. But they have launched their own learn.sparkfun.com site so I have high expectations. In Sparkfun's favor, though, is their public class offerings to get hands-on. .
No one site has the perfect mix of information and products. If Digikey et al. ever documented every part in their catalogs to current expectations, it would be nirvana but do not expect that anytime soon (although keep an eye on Element14). Until then, those sites which focus on the "right mix" of offerings and consumer information may just have the best customer base.