[PledgeBank] is a bulletin board where individuals can post pledges that will be "called in" or activated only if enough other participants also agree to the specified action.
For example: If 99 other people pledge to plant a tree on Sep 13, I'll plant a tree. If we don't get the full 100 pledges by Sept 12, none of the pledges will be called in.
FundableDotOrg is similar, except that it is about group purchases or fundraisers rather than pledges for action.
Interesting, this seems to be similar to what I was discussion at the CivicActions wiki site. http://www.civicactions.org/cgi-bin/wiki.pl?MarkDilley