This online onomatopoeia encyclopedia aims to teach Japanese onomatopoeia using illustrations, videos, and English equivalents.
Each onomatopoeic word is categorized into one of more than twenty-five different categories. Clicking on a word brings up a detailed page that includes pronunciation with audio, a short explanation, tags, example sentences, and images. These images either directly use the term or represent what it's used for (or with). Now and then Onomatopedia tosses in short manga pages, as well as further details explaining the usage of these terms versus other similar terms. All of this is provided both in English and Japanese.
You can search the site for specific words, choose by category, or list all the words that share tags. Some words span multiple categories and usages: words like ぶつぶつ, まったり, and ぴちぴち, for example, have multiple tabs with information for each usage.
Some entries also include YouTube videos explaining similar concepts, sounds, and how to use them. (There's also a monkey puppet that appears in them, if you're into that sort of thing.)
We do have one concern: none of Onomatopedia's images seem to cite where they come from. Some may be open-source or created by Imaginary Sound Creation, but because the site uses so many different styles of art, comics, and photography, we're a little wary. We hope they obtained them with permission and were curated specifically for the site.
Regardless, Onomatopedia is a useful tool that lets us all get ぺらぺら together!