This page aims to document the Unicode coverage of all the fonts (including non-free ones) in Debian. It was created using fontforge, font-manager and gucharmap. You should also read the DebianInstaller/GUIFonts page to find out about the fonts used by the installer.
Please add any fonts which might fill in some of these blocks to the Fonts/Missing page.
Quick test
Firefox seems to render all Wikipedia languages:
http://meta.wikimedia.org/wiki/List_of_Wikipedias
http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/languages/Names.php?view=markup
Incomplete blocks
Arabic extended-A: U+08A2 to U+08AC, U+08E4 to U+08EF and U+08F3 to U+08FE missing
Bopomofo extended: U+31B8 to U+31BA missing
CJK symbols and punctuation: U+302A to U+302D only available in unifont
Cyrillic extended B: U+A674 to U+A67B and U+A69F missing
Enclosed alphanumeric supplement: U+1F14F, U+1F16A, U+1F16B, U+1F18E, U+1F18F, U+1F191 to U+1F19A and U+1F1E6 to U+1F1FF missing
Georgian: U+10C7, U+10CD and U+10FD to U+10FF missing
Georgian supplement: U+2D27 and U+2D2D missing
Halfwidth and fullwidth forms: U+FFA0 to U+FFBE, U+FFC2 to U+FFC7, U+FFCA to U+FFCF, U+FFD2 to U+FFD7 and U+FFDA to U+FFDC only available in unifont
Lao: U+0EDE and U+0EDF missing
Latin extended D: U+A793, U+A7A0 to U+?A7A9, U+A7F8 and U+A7F9 missing
Miscellaneous symbols and pictographs: U+1F540 to U+1F543 missing
Mongolian: U+18AA only available in unifont
Musical symbols: U+1D159 missing
New Tai Lue: U+19AA and U+19AB missing
Small form variants: U+FE58 only available in unifont
Supplemental punctuation: U+2E32 to U+2E3B missing
Tibetan: U+0F8C to U+0F8F and U+0FD9 to U+0FDA missing
Tifinagh: U+2D70 and U+2D7F missing
Completely empty blocks
Some have glyphs available in unifont (*).
Arabic mathematical alphabetic symbols (U+1EE00...)
Avestan (U+10B00...) — free font available
Balinese (U+1B00...) *
Bamum (U+A6A0...)
Bamum supplement (U+16800...)
Batak (U+1BC0...)
Brahmi (U+11000...)
Buhid (U+1740...) *
Chakma (U+11100...)
Cham (U+AA00...) *
Common Indic number forms (U+A830...) — non-free font available
Imperial Aramaic (U+10840...)
Inscriptional Pahlavi (U+10B60...) — free fonts available
Inscriptional Parthian (U+10B40...) — free font available
Javanese (U+A980...)
Kaithi (U+11080...)
Lepcha (U+1C00...) *
Limbu (U+1900...) *
Lisu (U+A4D0...)
Mandaic (U+0840...)
Meetei Mayek extensions (U+AAE0...)
Meroitic hieroglyphs (U+10980...)
Meroitic cursive (U+109A0...)
Miao (U+16F00...)
Ol Chiki (U+1C50...) *
Old South Arabian (U+10A60...)
Old Turkic (U+10C00...)
Phags-pa (U+A840...) *
Rejang (U+A930...) *
Rumi numeral symbols (U+10E60...)
Samaritan (U+0800...)
Sharada (U+11180...)
Sora Sompeng (U+110D0...)
Sundanese (U+1B80...) *
Sundanese supplement (U+1CC0...)
Syloti Nagri (U+A800...) *
Tagalog (U+1700...) *
Tai Tham (U+1A20...)
Tai Viet (U+AA80...) — free font available
Takri (U+11680...)
Vedic extensions (U+1CD0...) — non-free font available
Also Klingon uses U+F8D0 to U+F8FF in the private use area (see ConScript Unicode Registry), but there are no fonts for it in Debian.
Some links
General:
http://www.unifont.org/fontguide/
http://www.alanwood.net/unicode/fonts.html
http://en.wikipedia.org/wiki/Unicode_typeface
Specific scripts: