Got an iPod or similar music player?

Got Windows 7 on your PC?

(Harder question) Got a mini stereo jack to mini stereo jack cable?

If yes to all three, you can play your music player through your computer speakers. This is great if you happen to be travelling and are stuck in your hotel room (er, like I am at the moment): for the minimum extra equipment (the cable) you can listen to your music on your laptop without having to ensure your music library has been copied onto your laptop drive or carry around an external drive with it all on. OK, maybe it's just me, but I do not have enough room on my laptop SSD to fit my music on. Maybe when I win the lottery (or the pools) and can afford a 256GB SDD, or they come down sufficiently in price, all this will be behind me, but until then…

Plug one end of your cable into your iPod and the other into the microphone socket of your laptop. My hardware causes an interrupt and the driver asks what kind of device I've plugged in. I selected LineIn.

Go to the Recording Devices tab of your Sound properties dialog:

Right click on the active input device and select Properties. Click on the Listen tab (this is only available in Windows 7, note) and then click on the Listen to this device checkbox:

Click Apply and you will be able to listen to your music player through your laptop speakers (along with all the pings and dings from your email/IM/Twitter app).

I must admit this was hard to find at first. I got the hint from Larry Osterman's blog post on the topic where he calls it "Monitor capture".