I just recently left a job in similar circumstances to you (I wasn't the only developer, but there was really only two of us, so I had quite a lot of knowledge that the other guy didn't have (and vice versa, of course)).
In terms of the normal documentation stuff, it's important to document an overview of the whole system. Individual components are already documented in the code, but the interaction between components and why this does that or why this needs to talk to that component are important and not always easy to figure out just by debugging/looking at the code.
Then, for about a month before I left, every time I did something that only I could do, I wrote down exactly what happened, what I had to do, and why. This was usually a case of "there was a bug in xyz component, to fix it I knew to look in file abc because of X, then I had to do this, this and this".
Of course, I left my email address and phone number in case anything came up that they couldn't figure out on their own. I got a few calls in the first few weeks, but they slowly dropped off.