Saw a segment on TV about a website devoted to free antenna designs for wireless, including Wi-Fi. In particular, there was a design for some simple parabolic reflectors to place on the little rubber-duck antennae that are common on most home gateway/access point devices, such as my Linksys BEFW11S4 and its newer companion the SMC 2804WBRP-G.

So, if you have your access point in the corner of your home, and there are areas that are perhaps not well covered at the moment, these would make a really simple booster and for the price of a piece of card and some aluminium foil well worth trying before you invest in a second AP to improve the coverage.