kana.guru is a free, open-source website for transliterating romaji text to hiragana and katakana. The core engine runs in the browser itself using WebAssembly, and is based on my other project, kana.
Note that kana.guru doesn't send the input to any servers, as it runs locally in the browser.
A mirror is located at GitHub. Feel free to give it a star if you feel like it. :-)
TODO
Please send patches via mailing list. Issues will be tracked here.