I added a diary capability to Advogato today. More generally, I did hacked on the "filesystem database" code to add good support for large lists.
Basically, numbers 0-99 just get rendered as _00 to _99. Numbers 100-9999 get rendered as _a/01/00 to _a/99/99. 10000 is rendered as _b/01/00/00.
This lets me do queries such as "what is the maximum numeric key" and "give me the last n" quickly, even for very large numbers. Coincidentally, these are the most common queries.
I plan to use this for posts and comments, but they fit nicely in the account framework, so I did those first.