Now, through June 30, 2013, buy a select Makita 18V LXT cordless combo kit and you can get an extra 18V LXT Lithium ion battery for free. This is a mail-in offer.
There are six combo kits eligible for the free battery promo, three 2-tool kits and three 4-tool kits. Model numbers: LXT211, LXT218, LXT238X1, LXT405, LXT406, LXT407.
Buy Now(via Amazon)
This seems to be a reasonably good deal if you were planning on buying a new combo kit. You should be aware that it takes up to 12 weeks for processing and redemption, and that the promo is only good in the US. There is a limit of 10 redemptions per household.
Purchases must be on new kits (and not reconditioned), and it looks like you can buy the kits from any authorized distributor. We compiled a list of eligible kits on Amazon (link above), but this is not an Amazon-only promo.
Thanks to Brandon for writing in about this deal!