I've just fitted a Suntour SP12-NCX suspension seat post and can thoroughly recommend. It's a very fluid action with no jerkiness and I think as close to rear suspension as you will get on a hardtail.
A little expensive though I got mine on offer (along with the required shim) from Rose bikes, sadly the offer has now ended.
A little expensive though I got mine on offer (along with the required shim) from Rose bikes, sadly the offer has now ended.