LEXiTRON ™ (เล็ก ซิตรอน) is a Thai <-> English dictionary produced by the National Computer and Electronics Center, Thailand (NECTEC). There are two versions of the dictionary (sound / no sound) that you can download and install for free.
LEXiTRON is maintained by Thai staff and its users can suggest new words and definitions. This may explain how some of the more unusual words, spellings and sometimes vulgar slang has worked its way into the dictionary. Nonetheless, for a free dictionary the overall quality is quite reasonable.