If you’d prefer physical books, this online store imports quite a few from Hong Kong.[2]
A dictionary program for Windows/Mac/Linux that I make [3], and another for mobile that I don’t make.[4]
Finally, a list of other good resources from Cantonese Alliance.[5]
It's an offshoot of https://words.hk (which has a Chinese interface). AFAIK we're the only modern Cantonese-Cantonese and Cantonese-English dictionary that is both comprehensive (50k+ entries) and word (詞) focused (i.e. not only explaining meaning of characters).
It's a huge undertaking, we've been on this for almost 10 years, and probably almost half-way done.
FWIW words.hk has quite a bit of traffic (from native/near-native speakers presumably) due to being well indexed by Google, but cantowords.com is relatively new and could benefit from more attention :)
PS: some of our content has made way to Cantonese dictionary apps as mentioned in sibling post(s), but the web site contains the most up to date content (which does get updated on a day to day basis as editing work proceeds)
At the time, I heavily relied on another great Cantonese dictionary/resource site: http://www.cantonese.sheik.co.uk/dictionary/
In addition to the Jyutping Cantonese pronunciation, it also shows corresponding Hanyu pinyin pronunciation for Mandarin. There are also number of learning resources on the site. I love how you can mouseover any character to get a pop up to learn more about the character - it's meaning as a single character, pronunciation, etc.