Frederick is a very good place to satisfy a hunger for roadfood. Lots of options and lots of good restaurants, from holes in the wall to one of the best restaurants in the US. I've summarized the roadfood-iest places below:
Already Mentioned In This Thread: Barbara Fritchie House - Since 1910, this is the roadfood poster child. OK, but too many things like 'green beans from a can' for my taste.
May's - A little too casual with the quality for my taste, and virtually every option is breaded and deep fried.
Brewer's Alley - A reliable choice, typical "micro brewery in an old building" place.
BBQ: Blue Truck - Haven't been.
Famous Dave's - A chain. As mentioned, the food is pretty good though. Just wish it had less "flair".
Tauraso's - now Danielle's, haven't been since the name change.
Not mentioned above: Red Byrd - this should be on every roadfooder's itinerary. Opened in 1958 and now run by a few great ladies, this place does the stuff you'd expect - fried chicken, pork chops, etc. Maybe only lacks a signature dish. This is about 10 minutes west of Frederick.
Dutch's Daughter - this is really roadfood in a pretty package. Can't say I recommend it highly, but if you spend time in Frederick, you should get to know this place.
Liberty Road Seafood - about 10 minutes north of Frederick, I prefer this place to May's. It seems to offer more preparation options and the crabs have been top notch.
Black Hog BBQ - my favorite BBQ place, offering different BBQ styles, corn bread, slaw, etc. Great stuff.
Mountain View Diner - one of those NJ-style diners that has 1001 things on the menu, this place is priced a bit high but 'corrected' by the portion size if you're into that kind of thing. Note about the website: the Moutain View offers just that - but NOT like on the website!
Roy Rogers - Although a fast food chain, there's some
interesting history there, and the surviving chain is owned/operated right from Frederick MD (except the ones in highway travel plazas) with good quality control. They can make a decent breakfast, fried chicken and roast beef sandwiches. If you have to get fast food, this is a worthy option with 3 locations around frederick.