I know that Bristol board is usually used for catalog and book covers, tickets, tags and printing brochures. But as a student of architecture and an avid designer, I use this kind of paper for my scale models (which, for those who are not familiar with the term, is a smaller physical version of something bigger, like a building or an automobile). I currently have several sets of two- and three-ply bristol papers that are used as walls of my scale models. One-ply is almost translucent so I rarely use them.
Bristol, as it is commonly referred to, is a kind of paperboard which is ideal for tons of things and comes in a variety of colors.
I love using Bristol paper because it's very versatile. I buy a whole bunch of them and stack them on my desk so I have something nice to use in various illustrations, technical drawings and even 2D art forms. Actually, illustration boards are also fine in such purposes but I find it impractical for you can only use one side of it. On the other hand, Bristol board have two working surfaces, so basically, you can use either or both sides.
And because I normally use charcoal and crayon as my primary media form, vellum finish is more ideal for me. It has a moderate surface texture which is really apt for friction-based medium. Works well with pens, too! (But if you are more focused on using ink, choose the plate finish Bristol paper instead -- it's glass-like texture is better suited for ink.)
The current brand I use is from Springhill Vellum Bristol. This Bristol paper works like a charm. And it's very accessible, even online. Just go to Amazon.com and search for Springhill Vellum Bristol Gold.
One pack contains 5 sheets and weighs a total of 1 pound so it's not that costly even if you order online. Their gold colored is consistent all throughout the papers and it is the exact hue I needed, too!
I also found out it worked nicely when used in promotional materials (when I once ran out of specialty paper for our theater presentation). I have tried several other brands and they all worked fine, but honestly, I think this is the best brand around.