I'd like to get a list of the hash power of the major pools, solo miners, and if applicable undocumented hash networks. How can I obtain this information?
If this information already exists, how did that site or service collect that information?
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Such information is already listed on Blockchain.info.
You can track that information by tracking the created block amount through various means - some pools embed information about the block's origin into the coinbase transaction, others might use the same reward address each time. Alternatively, you could try tracking which pool relayed the block first if you are connected to all of them, or perhaps keep a track of each pool's statistics on blocks and shares to see which one claims ownership of the block.
How about http://blockorigin.pfoe.be which aims to provide a reliable source of information by scraping pool web sites to see which pool claims ownership of each and every block (hence avoiding the problem that Peter Wuille mentions).
See https://bitcointalk.org/index.php?topic=78391.0 for discussion.