This is a fairly recent development (last ~ 5 years, maybe?). Purchases of school supplies and clothing of up to $100/item are sales tax free; I think this was intended to help stimulate the economy and give people a break. It's for this weekend, Aug 5-7. Stores have discretion to drop tax from other items too as long as they (the stores) pay the tax. Some businesses have laptops tax free this weekend too, for instance.
There are 16 states doing this for back to school (click link to view). Virginia also offers sales tax holidays during other months for energy efficient items and hurricane preparedness.
I ordered a backpack on Friday from LLBean, but did have to pay tax on that (they are in Maine); but the sales price was well worth it ($54.99 on sale for $39.99 plus free shipping).
Any Virginians out here who DON"T have kids and need sneakers, coats, jeans etc....go buy them today!! You don't have to have a school kid to get this price break. Many stores are including "dorm stuff" as well, which is good for us, as we're sending one off to college in 2 weeks.