Oekaki is, basically, an online drawing program. It's a java applet that allows you to draw, color, anything you want, really, on a small, simple 'canvas', and then post it to a bulletinboard. Anyone can do it, usually, though some groups make you sign up.
There are many versions of the oekaki program, the two most popular being PaintBBS and OekakiBBS. OekakiBBS is a more advanced program that allows for multiple layers, while PaintBBS only has two.
PaintBBS is probably the most commonly found program in both the English speaking sites as well as the Japanese speaking sites. There is both an English and Japanese version of the program.. once you learn one, you can use the other without hassle.
Oekaki, means literally, 'picture draw' ('e' being 'picture' and 'kaki' being the verb 'kakimasu' which is to draw/write'. The 'o' is simply an honoriffic, of sorts.)
I've done many oekaki, and though it's difficult to learn, with practice, you can make some very very good things. It takes a persistant, talented artist to know how to take full advantage of the program to make some amazing images. I'm still learning, but many of my friends are simply incredible.
Though it's not .hack related, here is the oekaki I completed this morning:
http://www.merchantguild.net/pronpar/oe ... es/953.png