Hanyu Pinyin to Hanzi converter
Type hanyu pinyin in the text box. Follow each character's pinyin by the tone number (eg: "ni3 hao3"). If you omit the tone number, only words with the neutral tone will be found. Put spaces or punctuation between the pinyin of separate characters if they do not have a tone number.
Characters that are recognized will be converted to the corresponding hanzi (Chinese characters). If there are multiple matches, the highest frequency match will be black, and the lower frequency matches will be grey.
This uses data extracted from the Unicode consortium's Unihan database.
You can find out a bit more in the blog post about this.