Goldendict

I found quite a good free dictionary for Windows and Linux users called Goldendict (a replacement for Stardict, if you've ever used that). You can download it (free) from http://goldendict.org/download.php. You then need to add your Thai dictionaries, which you can get from Lexitron at http://lexitron.nectec.or.th/ There are other free Thai dictionaries around (including Lexitron's own) but I like Goldendict because, if you want, it scans constantly in the background and you can have it set up so that a double ctrl-c brings up a translation of any highlighted word - very useful.