Good online dictionary I like, I found Tovnah is a good one: http://dictionary.tovnah.com

You can add a search box feature to your firefox for this website, check it out:

• URL: http://dictionary.tovnah.com/admin/
• Go down and find: “Add Khmer Online Dictionary (KOD) to the Firefox search bar“
• Check screenshot:

You can search the translation now!

Thanks Tovnah.com for this great tool.