Provably Prune-able Outputs! See https://bitcoinfoundation.org/blog/?p=290. The idea is to support adding a little bit (80 bytes) of extra data to a transaction, while signalling to people who don't need to know about it (i.e. almost everyone) that they can happily ignore any such data and thus avoid downloading it.
This has the potential to allow new protocols to be built ontop of bitcoin. Mastercoin is one new protocol that uses the pruneable, provable outputs functionality. Learn more about Mastercoin here: https://github.com/mastercoin-MSC/
For those uninformed, "MasterCoin" is an impossible attempt at pegging Bitcoin to the USD (among other things). It of course doesn't work because there's a limited supply, which means that the prices are raised and you're no longer pegged. The whole setup is essentially designed by the creator to make him and his "donators" rich (buy some now and get 10% more! was the original post).
Due to the ponzi-like setup, the people who paid 4 million dollars for "MasterCoin" are very, very vocal when it comes to promoting it.
