An ancient Greek font must include three accents, two breathings, iota subscript, and the diaeresis (collectively known as diacritical marks), in every relevant combination. For rendering ancient scientific or mathematical documents, it also needs certain archaic letters which were used as numeral symbols. Greek with multiple marks is called polytonic, as opposed to monotonic, which has been formally adopted by the Greek government and uses only a single accent mark.
The simplest polytonic Greek fonts (such as TekniaGreek) assign the diacritical marks by themselves to certain keys, positioned so that they appear over/under the preceding character. This sounds straightforward in theory; but in practice, it is difficult to get the marks to align properly, since the vowels differ in width. Additionally, I notice that web browsers sometimes separate the letter and the diacritics depending on the size of the browser window and the position of the line breaks.
A more complicated way of rendering the font is to include every necessary letter/diacritic combination as a preformed character in the font file. This method is more "foolproof" as to appearance, but the large number of characters necessitates special provisions for typing the characters. Many such fonts are available, as are software utilities to enable the typing of the characters. Unfortunately, until recently there has been no standard for where to assign the characters in a font. For example, users of Teknia Greek, WinGreek and its offspring, SGreek, SPIonic, SILGalatia, and Athenian and the Greek Keys system cannot read each others' documents because the fonts use different character mappings.
What is Unicode?
Unicode® is an international standard for assigning key codes to the characters of potentially every alphabet and language system in the world, making all Unicode-based systems mutually intelligible. It uses a vast number of character codes outside the standard 256-character set that non-Unicode fonts use, which is why older browsers and operating systems may not be able to use it. The number of fonts available is growing; most of them are relatively large and contain multiple languages, not just Greek.
Operating systems that support Unicode include Windows 95+, NT 4.0+, and Macintosh OS X. Compatible web browsers include Internet Explorer or Navigator 4+. For word processing, Word 97 or above is required for Windows; Mac support is said not to be good.
On the next page are links to internet sources for a number of Unicode polytonic Greek fonts, with samples and more information. I have created the Aristarcoj font which includes all of the Greek chararacters, including numerals, and Coptic characters. When I first uploaded this site, the only other such font I found freely available was Cardo; see the next page for how the list has grown.
Unicode polytonic Greek fonts
Technical information about Unicode Greek on the web
Who's linking to Aristarcoj?
James Comas' site at the University of Missouri
Inscriptions of Aphrodisias Project at King's College London
Thesaurus Linguae Graecae Unicode test page at the University of California, Irvine
Amherst College's Information Technology site
Semantic Analysis Research Tool by Carsten Ziegert
See also Harvard University Center for Hellenic Studies
Nick Nicholas' Unicode Resources site (see also Diacritics and Numerals)
Alan Wood’s Unicode Resources
WAZU JAPAN's Gallery of Unicode Fonts (formerly David McCreedy's site)
The Greek/Coptic font page by Luc Devroye—updated link
Peter Gainsford's Greek Font Archive
Wilson Mar's Greek Language Resources
Classics Links by Bruce R. Magee
Neo_^'s web page
Wilson A. Ribeiro Jr.'s Ancient Greek site (Portugese)
Stefan Lücking's Unicode page (German)
ta meta ta phônêtika
Links to more Unicode information and Greek fonts
The definitive source, unicode.org
Unicode Polytonic Greek for the World Wide Web by Patrick Rourke
Sean Redmon's Unicode page
James Kass' site with lots of browser information
Classical Greek Fonts and Utilities by Matthew Robinson