The windows XP notepad is UTF-8 aware (well, at least for the ones I have seen), that is, it can read and write UTF-8 encoded text files (and render most characters correctly). Most modern text editors are UTF-8 aware, however, not all will display other encodings and some doesn't render all the characters available in UTF-8 (they just show them as boxes []).
Another thing you may encounter is the Byte-Order-Mark (BOM), for PHP, the BOM is ignored, thus you should not save your text files with BOM.
To display the characters correctly of different encodings, you need the specific code pages as well. The editor which happens to have most of the useful code pages is the emeditor.
EmEditor supports Unicode little endian, Unicode big endian, UTF-8, UTF-7, Baltic, Central European, Chinese Simplified, Chinese Traditional, Cyrillic, Greek, Japanese (Shift-JIS), Japanese (JIS), Japanese (EUC), Korean, Thai, Turkish, Vietnamese, Western European, and all other encodings available in Windows.
In addition, it can actually convert to and from different encodings correctly (so far). That is, if someone sends you Chinese encoded using GB2312 (which is the main encodings used in PRC), you can open this in EmEditor and then save it as UTF-8 correctly.
The Chinese, Japanese and Korean (CJK) encode their languages with different encodings due to political issues (yes, you get politics in character encodings). Further more, the Chinese and Japanese have 3 different encodings each (again, probably due to politics but not sure). Fo example, the GB2312 encoding is adopted by the PRC, GB5 encoding is usually found in HK and Taiwan, etc.