One of the early pioneers of online journaling, it's been through quite a few mutations over the years, even breaking up into two separate sites at one point (freeopendiary and opendiaryplus, though the naming methods changed quite a bit).
As of right now, it's relatively easy to sign up, and the 30k characters-per-entry limit is far more reasonable than their previous 7k limit. I've gone around and tried other sites, and while others have a lot of extra features that OD somewhat lacks, I've kept with OD largely for the comraderie that I haven't encountered elsewhere. (Your results may vary.)
