David Berlind recently
blogged about using JavaDB (aka Apache Derby) to provide the browser-based persistence needed for offline AJAX applications. The downside is that you need a JVM in the browser, but the upside is cross-browser portability. Oh, wait. Maybe the JVM requirement is an upside (for a second, I forgot I work for Sun).
Berlind didn't mention the
mozStorage project, which is adding the open source
SQLite database to Firefox. According to the
mozStorage docs, you'll be able to issue SQL queries from JavaScript in the Firefox 3.0 timeframe. The downside is that it's Firefox only -- or may that's the upside.