To display text that contains Unicode characters, you need the appropriate fonts installed on your system. Without that, the characters are usually substituted with an empty box. If you're using Windows, there is a good chance that you already have the Unicode version of Arial. However on Linux and Mac OS X, many characters can't be displayed out of the box.
While it is much better to have a Unicode font specially designed for your target language and script, there are a couple of so called Pan-Unicode fonts that try to include as many characters and scripts as possible. But those are typically not free.
However there are some notable exceptions. The DejaVu project has the best free sans-serif Unicode font in its family of fonts. The GNU FreeFont project is also interesting: FreeSerif is the best free serif Unicode font with nearly 9000 characters, FreeSans is also nice, but has only ~4000 characters. If you combine all the free glyphs, you get approximately 10000 characters.
Code2000 looks also interesting. It has many more characters and scripts than all the free projects together. It was a shareware, and could be registered after free testing for 5$. Unfortunately, the website ceased to exist and it is not clear if its creator is still alive. It is also not possible to register the font anymore. By the way if you know what happened to James Kass, or how I could contact him if he is still alive, let me know!
For the time being, shapecatcher.com uses the free Pan-Unicode fonts Dejavu Sans, FreeSans and FreeSerif (in this order if the same character is available in more than one font). Some character images are also rendered with Code2000. Some glyphs are also already included from the Symbola font, mostly newer pictograms and emoticons which are not in Code2000.
|contains the latest versions of the following fonts: Dejavu (2.33), Freefont (20100919), Symbola (6.05), CODE2000
|The free symbola font supports many Unicode symbols and emoticons. From Unicode Fonts for Ancient Scripts.
|The original zip file of James Kass Code 2000. This font is shareware, but can't be registered anymore, because the original website is no longer available.
If you are using the fonts, read the license included for each font.