I've also built up a pretty big pile of fans... I've got two Zalman ZM-F3 fans, two SilenX Effizio fans, a NZXT case fan, the two fans that come with the XSPC kit I bought, a 200mm fan I bought for my old case, and one other I can't remember. :/
I'll never use them, but I can't bring myself to throw them away; "What if a friend needs them?!" if my excuse.