Client-side verification prevents the server from executing double-spending and other attacks. Automatic fail-over to solo mining allows the pool to disintegrate into individual mining nodes and to continue supporting the bitcoin network in a case of server unavailability, preventing a catastrophic slowdown of the entire network. The instant solo fail-over also works to increase miner efficiency by utilizing time that would normally be spent waiting for work from the server due to network lag or other issues.
BitPenny uses a delayed pay-per-share (CPPSRB) payout model that is immune to pool-hopping. Shares are paid in last-in-first-out order and never decay in value, with transaction fees included in order to help avoid large backlogs of unpaid shares. More details can be found at the BitPenny website.
BitPenny is the second-oldest mining pool, first available for beta testing starting February 08, 2011. It was the first pool to introduce the Pay-Per-Share model, instant payouts, and address-only connections that do not require registration or a password.
The IRC channel irc.freenode.net/bitpenny #bitpenny (webchat) offers conversation with fellow miners and information from BitPenny's operator.