It should be noted that there are two sorts of Gallifreyan, circular and linear. Most of the online constructions are circular (because circular is cool), but I did find one version of Old High Gallifreyan. OHG actually appears to be relatively common in the Whoniverse and is arguably more oft used in practical communication than circular systems.
Note: All ‘official’ Gallifreyan is NOT a full writing system and I’ve thrown my collected links dealing with it down in the miscellany section. :)
Sherman’s Circular Gallifreyan:
Bold, with a simple compounding way of translating and building sentences, the results of Circular Gallifreyan are visually imposing and impressive. This is a system designed to make the writer’s original language look bad-ass letter-for-letter. It is dependent on the alphabet of the non-Gallifreyan writer. Created by Loren Sherman. Known on Deviantart as BlackHatGuy.
Sherman’s Gallifreyan is exceedingly popular, because it’s both dramatic and practical from an aesthetic viewpoint. Because of this, there are a lot of variations upon the theme, like: http://elementairemoncher.tumblr.com/post/13786841662/gallifreyan-again-based-on-loren-shermans-how
ETA (10/2/2012): Funnily enough, Sherman’s has been, basically, adopted by most of tumblr, though people’s personal preferences for how it’s written changes somewhat. I write mine somewhat differently than Malallory, for example. Anyways, here’s another resource for this type in a somewhat easier-to-nom configuration: http://timeturners.wikidot.com/circular-gallifreyan
Greencrook Gallifreyan has little curls and swoopy bits, tiny bubbles and appears quite delicate and poetic. This version of Gallifreyan has unique rules and is interpretive, designed to be artistic as well as functional. This allows for Gallifreyan words and phrases to be translated several different ways from one root source and leaves a lot of translation freedom for the writer and reader. This is a symbolic language, where concepts (love, joy, wonder) are pulled from a lexicon. Created by Tumblr user Greencrook, known on Deviantart as Lewiscrook.
Greencrook’s Gallifreyan has also gotten a lot of attention and some interesting riffs, as 7eye’s translations show, as they reflect compound concepts forming simple words: http://7eye.tumblr.com/post/12214683664/a-few-more-gallifreyan-designs-cyber-and
WS Simple Gallifreyan (Font):
For a straight-up font for your computer: WS Simple Gallifreyan. Designed by Wiccked Stepmother Fonts, Melanie Cook.
DrawlingNell’s Circular Gallifreyan:
DrawlingNell’s Circular Gallifreyan is a representation of Gallifreyan that depends on strong evocative elements in circular form. It is not a system designed for translation so much as bold iconographic statements.
Old High Gallifreyan:
The first linear version of Gallifreyan, it’s also the original version of Gallifreyan represented in canon as far as I can tell. It is a chaotic writing system in that it was never codified for the show and therefore has been variously represented every time it was needed.
Freakism on tumblr parsed out a linear Gallifreyan version that follows the show’s Old High Gallifreyan, but is actually functional and not gibberish. This is a phonetic (more or less) version of linear Gallifreyan than can be used to translate from the writer’s native language.
BrittanyBGood’s Circular Gallifreyan (aka Doctor’s Cot Gallifreyan):
A Graphic Design major, BrittianyBGood’s version of circular Gallifreyan is both solid, bold, and entirely phoentic, using overlapping circles that represent sounds.
It is currently under development. (Edit 3/26/2012): Finished and updated.Of the writing systems detailed here, it’s the closest to the circular Gallifreyan represented in the show.
Collective Gallifreyan (The Gallifreyan Conlang Project):
The Gallifreyan Conlang Project is an exercise in linguistic nerdery of the highest order. It is the most comprehensive Gallifreyan I’ve set my hands on. It uses the reading direction ‘counter-clockwise from the bottom’ as Sherman’s Gallifreyan does, however the alphabet is phoentic, and each ‘letter’ is based upon a phone, so that a transcription into Gallifreyan is both a written word and a road-map on how it’s pronounced.
(Edit 3/3/2013): Collective Gallifreyan has moved along at a pretty good clip and now has a huge lexicon as well as a number system and, as a conlang, has a bunch of really cool concepts being implemented (gender-inclusive and complex timestream descriptions being notable). The Collective Gallifreyan tumblr has also been demonstrating how their alphabet works by translating names phonetically. Examples can be found by following the link above.
Other miscellaneous Gallifreyan links:
The Gallifreyan language as detailed in the New Series Adventure books, which appear to be a random collection of symbols as well as the Time Lord’s base 7 number system. http://thedoctorinthetardis.freesmfhosting.com/index.php?topic=1705.0
An icon pack by Girl-on-The-Moon (DeviantArt name) and Lydiabutz (tumblr) for Macs, based off the official DW symbols from the New Series Adventure books: http://girl-on-the-moon.deviantart.com/art/THE-ICONS-OF-RASSILON-116296443
The doctor’s name as pulled from his cot, transcribed by DA user Sculcuvant: http://sculcuvant.deviantart.com/#/d3i372o
DA user Purplerhino’s photoshop brush set: http://purplerhino.deviantart.com/gallery/#/d2icvn8
Fascinating exploration of the difference between Linear and Circular Gallifreyan and how they interact. http://dimensionsintime.tumblr.com/post/15302997319/the-post-you-wrote-about-the-gallifreyan-can-be
River Song’s note to the Doctor, ‘Hello Sweetie’: http://green-eyed-hipster.tumblr.com/post/9519370680
Ten Rules for the Gallifreyan Language (from charamei on JournalFen) (These are awesome): http://www.journalfen.net/users/charamei/9487.html
Edited to add (4/12/2012): If you’d like to use a program to help you draw Gallifreyan, I put up a couple of suggestions on what you could use over here:
Let me know if I’ve forgotten anything! I’d love to keep myself up to date on what Gallifreyan is out there. :)