The giongo/gitaigo dictionary
The real name for this section should have been "onomatopoeia and mimesis", but not many people know what onomatopoeia are (or how to spell it =) and even less people know what mimesis are (but can spell that better than the first). Onomatopoeia (in Japanese 'giongo', 擬音語) are better known as "sound effects" and involve things like "kaboom" or "sploosh". Mimesis (in Japanese 'gitaigo', 擬態語) are "state words" and are things such as "wibble" and "sparkle". These words are an intrinsic part of Japanese much more than they are in western language, so a searchable dictionary for them is always a plus. This is that dictionary.
When used adverbially, these words are always followed by と and the verb they modify. Typically the construciton "to [...]" such as "to go boom" or "to sparkle" simply involves the construction [word]とする - note that because adverbial use is not the exclusive role of these words, none of these words can be found with と at the end. If you have a sentence 雨がザーと降ってきた then look for ザー, not for ザーと. This might seem fairly self-evident, as と is just a particle, but for some words such as はっと the と is typically considered "part of" the construction. Not in this dictionary.
On a usability note, I have chosen for the full stop [ . ] to act as glottal stop marker (which kind of makes sense... it's a stop, we already have a character in the western system to denote a stop, thus that character is used for a stop =). Consequently, if you wish to search for "あっ" then you can search for "a." and get the result you were looking for. For long vowels, use a dash [ - ], not the actual vowel. The reason for this is that giongo and gitaigo aren't "real words" in the sense that there are kanji versions for them, they're just written the way they sound or seem, so even in hiragana typically the long vowel marker is used rather than doubling vowels with actual vowel characters. If you want to find "ずう", then look for "zu-".