Lexitron
A Thai-language site that offers not only an instant Thai/English translator tool, but also games and other useful tools. Each definition includes any synonyms and a sentence that uses the word in context.
http://lexitron.nectec.or.th/index1.php

Stars21
World Star Free Dictionaries and Search Engines site is not a mere collection of web pages, but a heavy software program composed of hundreds of World Best Dictionaries and Searches. The site includes an instant Thai-English translator, as well as search engines from five continents.
http://www.stars21.com/dictionary/English-Thai_dictionary.html

Dicts
On this Internet site, you can find a free online bilingual dictionary that allows you to translate Thai words into English.
http://dicts.info/2/english-thai.php

Freelang
On the Freelang site, you will find a free translation dictionary from Thai to English, which you can consult online or download.
http://www.freelang.net/online/thai.php

Logos
Logos bills itself as the largest freely accessible multilingual dictionary on the web, with dictionaries in 232 languages. Thai words and phrases can be translated into English, and vice-versa, at the touch of a button. Logos also features a universal conjugator, children’s dictionary, and more than 43,000 online books in over 100 languages, including 4 in Thai.
http://www.logos.it/

Ectaco
This site offers instant translations of Thai and English words. This free Thai-English dictionary contains 400,000 words.
http://www.ectaco.co.uk/English-Thai-Dictionary/