文字轉換拼音工具 Tool for converting characters into pinyin
There are two websites providing great service, you can choose the one that suits you,
拼音字典+標注 7.0 (test)
I always use this one, as it not only converts characters to pinyin but also creates a *.open file format of all the pinyin and characters. (example of the *.open file format)
Another website
http://zhongwenzhuanpinyin.51240.com/