This thread was the primary information source on alt coins for me: https://bitcointalk.org/index.php?topic=134179.0

I like the idea of PPcoin very much, being an ecologic alternative to Bitcoin. They still have mining, like Bitcoin and all other alts, and this will consume as much electricity. But when all coins are mined, the power needs will drop significantly. The creators devised a very clever way to record transactions in the block chain without the need to do all the calculations which are done when mining.

Website, including the white paper with a detailed explanation: http://www.ppcoin.org/