Community

P2Pool Discussion board: https://bitcointalk.org/index.php?topic=18313.0
Note: There is a very active group of dedicated p2pool'ers on this discussion board that freely offer their knowledge and help. This is the first and best place to look for p2pool advice and support.

Info

P2Pool wiki: https://en.bitcoin.it/wiki/P2Pool
Official p2pool site: http://p2pool.in
Historical p2pool mining info: http://p2pool.org/stats
Setting up merged-mining (Ubuntu): https://bitcointalk.org/index.php?topic=651819.0
Single script setup of p2pool node with all supported merge-mined coins: http://bitcoin.kyros.info
Original p2pool merge mining setup thread: https://bitcointalk.org/index.php?topic=62842.0
Comparison of p2pool, Eligius and BTCGuild: https://bitcointalk.org/index.php?topic=416933.0
Relay Node: https://bitcointalk.org/index.php?topic=766190.0

Source Code

Main GitHub: https://github.com/p2pool/p2pool
Forrest's original p2pool source: https://github.com/forrestv/p2pool
Rav3n's fork (lots of alt coins): https://github.com/Rav3nPL/p2pool-rav
Roy7's vardiff patch: https://github.com/roy7/p2pool/commit/71121fa79016db7679ca0dcbf4328d9c9d51c97d
Matt Corallo's Relay Node: https://github.com/TheBlueMatt/RelayNode
Matt's fork of p2pool with relay node built in: https://github.com/TheBlueMatt/p2pool

Front Ends

Extended Front End: https://github.com/hardcpp/P2PoolExtendedFrontEnd
roy7's p2pool node status: https://github.com/roy7/p2pool-node-status
Justino's "Punchy" Front End: https://github.com/justino/p2pool-ui-punchy
Norgan's: https://github.com/norgan/p2pool_fancy_front_end