SatoshiBitcoin

SatoshiBitcoin Core now uses 'headers-first synchronization'. This means that we first ask peers for block headers (a total of 27 megabytes, as of December 2014) and validate those. In a second stage, when the headers have been discovered, we download the blocks. However, as we already know about the whole chain in advance, the blocks can be downloaded in parallel from all available peers.

In practice, this means a much faster and more robust synchronization. On recent hardware with a decent network link, it can be as little as 3 hours for an initial full synchronization. You may notice a slower progress in the very first few minutes, when headers are still being fetched and verified, but it should gain speed afterwards.

A few RPCs were added/updated as a result of this:

getblockchaininfo now returns the number of validated headers in addition to the number of validated blocks.
getpeerinfo lists both the number of blocks and headers we know we have in common with each peer. While synchronizing, the heights of the blocks that we have requested from peers (but haven't received yet) are also listed as 'inflight'.
A new RPC getchaintips lists all known branches of the block chain, including those we only have headers for.

Transaction fee changes

This release automatically estimates how high a transaction fee (or how high a priority) transactions require to be confirmed quickly. The default settings will create transactions that confirm quickly; see the new 'txconfirmtarget' setting to control the tradeoff between fees and confirmation times. Fees are added by default unless the 'sendfreetransactions' setting is enabled.

Prior releases used hard-coded fees (and priorities), and would sometimes create transactions that took a very long time to confirm.

Statistics used to estimate fees and priorities are saved in the data directory in the fee_estimates.dat file just before program shutdown, and are read in at startup.

New command line options for transaction fee changes:

-txconfirmtarget=n : create transactions that have enough fees (or priority) so they are likely to begin confirmation within n blocks (default: 1). This setting is over-ridden by the -paytxfee option.
-sendfreetransactions : Send transactions as zero-fee transactions if possible (default: 0)

New RPC commands for fee estimation:

estimatefee nblocks : Returns approximate fee-per-1,000-bytes needed for a transaction to begin confirmation within nblocks. Returns -1 if not enough transactions have been observed to compute a good estimate.
estimatepriority nblocks : Returns approximate priority needed for a zero-fee transaction to begin confirmation within nblocks. Returns -1 if not enough free transactions have been observed to compute a good estimate.