The Sixth Discipline, which has been free in the US Kindle store since last summer, has been holding steady at about the 5,000 rank in the "freebie" part of that Kindle store. Today, for some reason I can't understand, it jumped to 1,077, and it's #30 in science fiction freebies.
Wish I knew why! I did a Google search but couldn't find anything. But I'm not complaining. Just hope it lasts for a while.
Update: As of 4/18/13, The Sixth Discipline is up to #13 in free science fiction and #652 in overall free Kindle books.