Unicode site is well-known. But sometimes it's not enough. Here are some more links.
I always need information about language-specific glyphs. Now I've found notes on Cyrillic and East European languages. One of the pages announces itself so:
Does your web browser properly support Cyrillic and East European encodings? Check it!
And there is also a summary page that contains Czech and Slovak characters, Polish characters, Romanian characters, Croatian and Slovenian characters, Hungarian characters, German characters, Russian alphabet, Special Byelorussian and Ukrainian characters, Special Serbian and Macedonian characters.
Another sort of information I need is codepages. Microsoft-specific data can be found at URLs like these:
Changing the number in URLs, it's possible to get all required codepages data.