Devifoxx of Quetzacoatl (with a some help from me and Nicolette) created a very well documented (pics for all mods) and organized collection of just about every custom .DAT we could find publically available. If you would like this collection, you may download it
here (48.4mb). If you have mods you would like to contribute, feel free to submit them to Devifoxx or me (easiest to submit them to Devi since he manages the collection). Email addresses are included in an text file within the collection. I hope this is a big help to all those interested in giving their ffxi a unique appearance.
Edit(11/4/05): So you don't have to flip through several pages, you can find both links here-
DAT Collection Download Devi's webpage
Edit(11/30/2007):
Mod Community website with guides