I don't know how good it is, but this is one I use.. http://dictionary.site.lv
http://translate.google.com also offers alternatives and examples when you translate..

Yes....http://translate.google.com  (http://translate.google.com/#en/lv/) is a good dictionary

"+" http://translation2.paralink.com/

I will not agree about the translate.Google. Perhaps for separate words it can give some answers, but to translate whole texts either way (en-lv or lv-en)... That will just yield some totally garbled text from which only someone who already knows both languages can make any sense. Latvian grammar is just too complex for it. Sorry, I cannot offer good alternatives, but I just wanted to warn against total trust in that translator. Especially when I've seen people trying to pass those translations as knowledge of language when it's so obviously wrong.
P.S. Try to get the hand dictionary if you can. Although again, it will not help you much with grammar and sentence structure.