I love how much information McMaster gives you. Mechanical drawings and 3D CAD models of course, and also descriptions of how things are used, the properties of different materials, how to size stuff, etc. All the basic things you need to know to design something.
McMaster-Carr's fulfillment services are truly top-notch. There have been times that I've specifically ordered from them because I knew that I'd have it the next day. They've never failed to impress me with their service.
Their physical catalog is one hard catalog that I really miss! Despite the fact that the website undoubtedly offers more product than the multi-thousand paged hard edition, I remember many projects in my college days inspired by simply flipping through it's pages.
As a studio art major in another life, I can't recommend McMaster-Carr enough! I was doing a lot of metal fabrication, lighting, and plexiglass work, I can't imagine finishing my thesis without their selection and speed. Not to mention all I learned just by reading their great descriptions and images.