I was lucky enough to grow up on Bangkok Delight of Columbia, MD as my source for Thai food. http://www.bangkokdelight.com/
That place is constantly winning awards (and in Columbia where people rarely use their own kitchens that is quite a feat).
My only problem with them is that they won't make their food spicy enough. I ask for extra spicy, but it's never hot enough (probably because I'm a whitey and they don't think I can handle it), so I always have to ask for chili paste and chili powder on the side.
There's another one I go to in Greenbelt (it's like 5 min from my house) called Siri's Chef Secret. The outside looks kinda shabby, but the inside is actually quite elegent looking with a huge fish-tank and a well stocked bar. The food is good, nice atmosphere. It's a bit preicey, but the portions are large and they give a 10% discount on dinner to University of MD students.
A new Thai place just opened up in downtown Silver Sprung above Eggspectation. I plan on trying them out this week or next.
Try this website to find resturaunts closer to you.
http://www.thai-food.com/restaurant_directory/usa/usa.html

The place to go for good, inexpensive Thai in DC/MD is Wheaton. There are several good places right around the intersection of Georgia and University Blvd.

The best of these places is probably Ruan Thai, a tiny hole in the wall with amazing food. One of my favorite dishes is a spicy salad with grilled eggplant and a sprinkling of ground pork.

This is the review from Tyler Cowan's Ethnic Dining guide:

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Ruan Thai, 11407 Amherst Ave., Wheaton, 301-942-0075, not so far from Rt.29 and University Blvd.

My Thai restaurant of choice, after Duangrat’s. Not just the usual stuff. Fresh ingredients, and truly spicy. When it comes to ordering, you can’t go wrong. This place is exactly what a Thai restaurant should aspire to be.
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Another one you might want to try in the same neighborhood is Suporn's, which makes their curries with chunks of pineapple.

I tried out a few appetizers at that new Thai place in downtown Silver Spring and was not impressed. There couple of other new-ish Thai places near there (Pinto Thai and Thai Derm) that I've heard better things about.

If you want spicy, try Suporn's in Wheaton. If you ask, the chef there will make the food hot enough to burn your face off.

There were only two Thai places I've been to or ordered from that were remotely memorable. 4912 Thai on Wisconsin, and some place in Bethesda I unfortunately can't remember, which is a shame because it was the better of the two. Also, though it's not strictly a Thai place, I really like the pad thai at Spices in Cleveland Park.

Beyond that, I'm kind of resigned to thinking of Thai as being like Chinese: enjoyable cookie-cutter takeout fare that is rarely terrible but rarely great. It's lost some of its luster since I've gotten into Vietnamese food, which is lighter and more subtle.

I agree that the new Thai at Silver Spring (the one above Eggspectations) is unremarkable. The only surprising thing about the place is that they have no lunch special-type pricing in place, making it hard to get away with paying less than $15/person for lunch.

However, I've had two meals in the past two weeks from Pinto Thai, a hole-in-the-wall joint on Georgia Avenue that apparently is in the same space formerly occupied by Thai Spice, a restaurant that no one missed when it closed. Pinto offers a lunch buffet reminiscent of typical strip mall Chinese restaurants, but with surprisingly good quality food, including an all-you-can-eat option for just $7.25. Their Panang Gai and their Gai Pad Kaprow are both excellent, and co-workers give good reports on their Green Curry, their tofu dishes, and other menu items. I'd definitely go there again before I'd go back to TaSS.

The last Silver Spring option, Thai Derm, is in no way new, they've been there for at least eight years, probably longer. Their food is decent enough, but their location (wedged into the ground floor of two adjacent town houses) is uninviting and kind of plasticized. Not bad for takeout, I suppose.

The best place for Thai, by far, is Thai Square in Arlington (Columbia Pike and Glebe). Nothing else really compares to this place. Good "in town" Thai would be Kanlaya in Chinatown. The Wild Pork is my fav.

I actually ate at Thai Square for the first time last night. It's quite good, and they have a few dishes you won't find at most other places. I'd rate Ruan Thai as slightly better, though.

Thai Derm is eight years old? Wow… I guess I hadn’t been to that part of Silver Spring for a long time before the recent revitalization.

Another Thai place worth trying is Rice, which is near Logan Circle and has a non-traditional take on Thai cuisine, including an entire menu of items with green tea in them. Some of it is pretty good.

I wanted to alert everyone to watch out for the waitstaff at Thai at Silver Spring. Recently friends and I went there for dinner and had a really nice time. The food we thought was great, service seemed okay. Our bill was over $100 and we tipped handsomely. There were 4 of us and we split the bill 4 ways, one paying her portion by credit card, the rest in CASH along with the tip. A few days later, one friend discovers that th entire amount of the bill was charged to her account and the money pocketed. Though she signed a charge receipt for the correct amount, that amount never showed up on her account, only the amount for the entire cost for 4 people. When they called the management, they were treated rudely and the amount has yet to be returned to her. Even with a return visit to the resturant to discuss, the mananger began to yell at the customer rudely and accuse her falsely. It was a harrowing expereince, not to mention being robbed of over $100! Her next step is 7 On Your Side for a resolution to this. Watch your credit statements when you go there.

Hey Brittany, Tell your friend to just call up her card and report the fraud - especially if she has her version of the signed receipt. That's what card companies are *there for*.