One of the coolest kid seats I've seen is what this dude did for his Xtracycle.
Another I saw that looked pretty cool and was recently reviewed was the iBert safe-T-seat.
Oh, and BTW, I can confirm that when they say max weight 75 lbs or whatever, that might mean a child but is equally applicable to a pony keg of beer. Those harnesses might be good at holding kids (dunno) but also just peachy at keep your Fat Tire from rolling out, too. :-)