What services let you trace the history of Bitcoins? What services let you trace the history of Bitcoins? For instance, if the goal is  to trace all transactions associated with Bitcoins owned by a certain individual or company, what's the best way to accomplish this?
WalletExplorer.com can associate addresses and Txs with ~100 services, including:


LocalBitcoins 
Silk Road, Agora, other DNMs
Defunct services like Vaultofsatoshi, Bitcoinica, Inputs.io, Mt. Gox etc
Strongcoin, other wallet providers
Lending platforms like BTCJam
BTC-E, Coinbase, Bitstamp and other popular exchanges
Gambling sites like Just-Dice, SealsWithClubs Poker
Mixers/tumblers like BTCFog


I'm extremely curious how this is done, to be honest. I suppose it's just tracking UTXOs of known addresses and identifying associated Txs, but it's quite fascinating how accurate the data is.

The service doesn't identify newer addresses, of course, since fresh addresses need x blocks / UTXOs to deduce the associated account/marketplace/exchange.