I have updated  for my own purposes the ActiveRecord library from CI 1.7.0 so you can use it in a standalone mode

The original version was downloaded from http://hasin.wordpress.com/

You can download it at http://www.documentopia.com/downloads/

I had to remove a few things so test it out before you use it, my testing only extended to selects, inserts and truncating tables.   It really needs some error handling added in if you are doing anything but dead simple queries.