One of the best Hmong (and Mong) dictionary resources, Jay Xiong's Hmong-English Dictionary Software, is now available for free (although, a $5 donation is suggested). It not only includes the ability to search definitions in both Hmong and English, but also provides sound clips of consonants, vowels, and tones. You can also search the dictionary online at his website and hear sound clips of common words. The print version of the dictionary is also worth the price.
Jay also promotes Tony Vang's Hmong Tutorial that includes a book and software that covers the basics of speaking and writing Hmong. I haven't used it myself, but it sounds like a good place to start for people who want to start learning Hmong language.
It's sad to see that the Saturn School White Hmong Dictionary has been taken offline. Along with the dictionary, it also featured folktales and proverbs--useful information for people interested in Hmong culture. Koua Lee's online Lomation Hmong Text Reader is still available and provides both a sound file of the word and a definition. (Type in the word, or copy and paste some text and click 'Read'. Moving the mouse over the word (after clicking 'Read') causes the definition to pop up.) It's especially handy for getting a quick sense for a sentence since it can pull up definitions for several words at once.
For those interested in a print dictionary, Yuepheng Xiong's recent Hmong-English/English-Hmong Dictionary is probably the best available. You can find it at his book store (ABC Hmong) in both pocket and standard size (although the standard size doesn't appear to be available online at the moment).
I've recently come across a couple of new online Hmong dictionaries, but I won't link them here since it appears that they have scraped their content from other sources without attribution (not to mention, both featured several incorrect definitions). I even contacted the owners of the website to voice my concerns about their potential copyright violations but received no response. You can find my updated collection of Hmong dictionary links here, which includes link to specialized dictionaries for law, health, and religion.