How can I work with multiple testnets? Apart from running a private testnet-in-a-box there are also the public testnet instances. You can use this instead of the regular (production) bitcoin network by setting testnet=1 in your bitcoind configuration file. However, the fact that there are multiple instances and not just one testnet can be confusing.

Is it my version of bitcoind that determines which testnet I am on, or something else?

How can I see which testnet I am using?

Is it possible to switch testnet? If I was on testnet 2 and wanted to use testnet 3 would it require upgrading bitcoind? Would upgrading bitcoind automatically bring me over on the new testnet, deleting all my old blocks, transactions and coins from the old one?

Are there other bitcoin clients that support testnet and if so which one(s) do they use?

If I want to use blockexplorer or something similar, do they only support the "latest edition" testnet?
If you are using Bitcoin version 0.7, you are on testnet 3. Updating to that version will put you in the newest testnet. Blockexplorer and Testnet Faucet use that version.