Montgomery County fair in Gaithersburg, MD runs from August 9th to the 18th. There is tons to see and do there, especially if you have kids. The first can't-miss event is the pig/sheep/duck/goat races. These little critters race around a track to the cheers of the crowd. Lots of people go so plan on getting there 5 minutes early if you want a good view.
Right next to the race track is a stage where they have different shows. I saw Michelle's magic poodle show (suprisingly, not a magic show) and Agricadabra (not surprisingly, a magic show)
Old MacDonalds Barn is another great place to go, they have various animals such as a lama, an alpaca, sheep, goats, horses (including two miniature horses), baby ducklings, and piglets. This year the piglets are really small and cute.
In the evenings at the grandstand there are events such as tractor pulls, demolition derbys, and monster truck shows, there are also numerous free events throughout the day. Take a look at the schedule of all events.