I've been tracking my small updates to TKPal at Simpleform but I should mention here that Grouphug.us has gone and implemented TKPal site-wide. Pay once and get access to comment on any confession you wish.
Bill Zeller sent me a whopper of an update tonight that abstracts the database with a nice class so a number of databases will be able to be plugged in. My next goal is to get SQLite running with that class.
Once TKPal development has settled down, I'll be rolling out some TKPal uses. One of the great things about the system is it gives people who have thought of implementing some kind of PayPal functionality on their site some real code they can extend—all for the bargain price of $1.