The Diacritics site (http://diacritics.typo.cz/) has been around for a while, but hadn’t hit my radar until recently. What it does is describe the typographic design of individual diacritics (accent marks).
There’s especially good coverage of the Central European accented letters (e.g. z̧,ł,ǫ) with ogoneks, hacheks and others. Site owner Filip Blažek is Czech, so he definitely knows his Central European typography. The site includes links to some excellent articles on proper diacritic design.
Coverage also includes Turkish (Ỉ,ş), Vietnamese tone marks, Greek diacritics as well as the familiar Western European accents (ñ, é, ü) and some Sanskrit. Finally the site includes articles about diacritic use and design in individual languages. There may be gaps, but readers are invited to supply more information.
On a personal note, I would add that most English-based typographers probably underestimate the importance of making diacritics legible – hence they often look like afterthoughts. As a result, I am forever adding bold formatting to accented letters on Web pages, just to enhance legibility.
Postscript – May 17, 2010
Another recommended site is http://www.eki.ee/letter/ out of the Eesti Keele Instituut in Estonia. It’s a more technical database interface, but has lots of useful information, particularly in terms of specific languages and encoding tables.