It is hands down the best implemented e-commerce site. Just ordered some SS stud anchors from there today. Didn't bother checking the hardware store for them.

Another distributor that is super strong on the fulfillment side is Digikey. I had a tour of their operation several years ago and it was amazing. I saw orders picked and moving to the UPS truck 10 minutes after payment. Web site not nearly as good as McMaster though.