How do I run my own testnet? What's testnet-in-a-box? If I want better control of my test environment while developing a bitcoin-related application it seems useful to run my own testnet rather than use the public testnet. How can I do this?

What is testnet-in-a-box? Can I use that? What are the advantages and limitations, if any? Are there other options?
Checkout the updated Testnet in a box.  You will need bitcoind already installed in your system.

The "Testnet in a box" linked above is just like the world-wide bitcoin network except the only members are the 2 running on your computer. There should be no difference except that it's easier to generate coins.