Let's face it, martial arts movies aren't really known to have a lot of substance. They are usually filled with cliches, unrealistic action scenes, and excuses for the lead actor to rip his shirt off for a fight scene. However, they can be fun diversions to see your art in another medium.
Tong Po and Kurt from Van Damme's "Kickboxer"...a terrible mess of a movie lol
Here are some movies out now that may get you in the mood for Muay Thai, or make you laugh at their ridiculousness. *This is not an exhaustive list...there are a lot of other movies featuring Muay Thai, but not very easy to get your hands on*
The obvious choice, and the movie that pretty much made everybody gain an interest in Thai fighting arts. It features some Mae Mai Muay Thai...ancient techniques not seen in sport Muay Thai. As far as the story goes, it's nothing special. Watch it for the unique stunts and fight choreography. Note that alot of acrobatics and wushu are mixed in to give the fights some flair...this isn't all really Muay Thai. If you haven't seen it, you should just for fun.
Tom Yung Goong (The Protector)
From the same people who brought you Ong Bak. Some more ancient techniques are used. Alot of people watch this movie because it features fights between Tony Jaa's character (representing Thai martial arts) and other types of fighters (Capoeira, wrestling etc...). People really eat up the whole "style vs style" fights thing. The story is ridiculous and so are some of the characters. Another movie you won't be watching for anything but the action. On that note, expect much less stunts but a lot more arm breaking.
Based on a true story about Nong Thoom, a male Thai boxer that experiences great success but then goes on to have a sex change. It provides a relatively realistic portrayal of Thai fights and fighters from Thailand training in the gym. Don't expect over the top action as much as the development of Nong Thoom's story from childhood to training to sex change. The action you do see is delivered from a different mentality: adding dynamic camera angles and cuts to realistic Thai boxing choreography, with some dramatization (music, rain, etc).
Chok Dee the Kick Boxer
This movie is a fictionalized version of the life/career of Dida Diafat, a French Muay Thai champion. It follows the protagonist's transition from crime (the movie starts off with him doing time) to traveling to Thailand to train and fight. It also has a more realistic portrayal of Muay Thai technique and fights, as well as the life of Thais in the gym. The story isn't anything special, but as a fan of the sport I actually enjoyed this movie the most out of the ones listed here (despite some obvious cheese and "expected" bits with Thai gangsters). Also: "Chok Dee" is the protagonists nick name, and means "good luck" in Thai.
There are other movies...Kickboxer (JCVD...bleh lol), Chocolate, Ong Bak 2...etc etc. But if you are just getting into Muay Thai and looking for some of the old standby's that are known to feature the art/sport, the movies I have listed are a good starting point.