Newegg is a computer parts retailer that ships quickly and accurately. They have very low prices, sometimes the lowest, and their catalogue is clear about the pricing (it shows which products have rebates and what is required to get them).

Their selection is very comprehensive, and the product search engine makes it very easy to drill down to find exactly the product you want (check out the memory section for RAM: it is easy to get lost trying to find the right combination of features, but their system builds on your previously-chosen requirements to narrow the search).

I highly recommend them for computer and electronics parts purchases.