The Grutt pass is one of Tokyo's best bargains. For 2000 yen, the purchaser is given a passbook full of coupons good for free entrance into over 40 Tokyo museums/zoos/aquariums,... Special exhibitions are not included, but discount coupons for those exhibitions are also included in the passbook. The pass is good for two months from the date of the first day of use.
This promotion was run in 2006 and 2007. I do not know if it will continue past March of 2008. http://www.rekibun.or.jp/grutto/