"Blockchain Explorer" software? I've been going to blockchain.info frequently to check addresses, transactions, blocks, etc. But since I've already got the blockchain on my hard drive, this seems kind of pointless. Is there any software similar to blockchain.info that runs natively on Linux, and uses Bitcoin-qt/bitcoind's blockchain, and therefore doesn't require an internet connection?
Abe does exactly what you want. I will walk the entire blockchain from bitcoin-qt/bitcoind and populate a local database. It supports several database backends including sqlite, Postgres, MySQL and Oracle.

It will take quite some time to walk the blockchain and populate the database, possibly a few days.

It has a web based interface that is very similar to blockchain.info and blockexplorer.