I've used FreeCycle in the past and it has basically worked well for me. A giver posts some item that they don't want anymore and potential receivers email the giver. An exchange plan is worked out, usually involving the receiver coming to the giver's house. I know that some groups suffer from too many no-shows, too much junk, and too large an area to easily drive and pick up items. I've been in one FreeCycle group that was wayyy to big, but my current area is quite reasonable. I've given away a lot of things and picked up a few items as well.
I haven't used BillShrink, but I did write about it. In that article, I also talked about a similar program called Validas. I didn't find any value in using Validas, other than to reaffirm my belief that I have an exceptionally well priced cell phone plan. (Sprint doesn't offer it anymore, sorry!) However, if you've never analyzed your cell phone bill, or you have multiple lines, or you are spending a lot on your cell phone bills, it would certainly be worthwhile to check it out.
I also haven't used SmartPig, but I have done some research into it. I see some big downsides: you have to authorize automatic debits until you reach your goal, interest only accrues quarterly (not good if you take your money out just before the end of a quarter, and there are limited way to receive your money when you are done. On the other hand, it is free, it pays a respectable interest rate, and it forces you to save. I can see it is a specific tool for a very specific goal, but not good for most savings applications.
I'm curious if anyone has used any of these other ideas – let me know!