I think it's been awhile since someone asked - what's the best free way to record gameplay for upload to Youtube? I'd like to get some of my Atari 2600 and obscure Coin-op gameplay uploaded. Thanks!
Some emulators have a built-in video recorder already, so you may want to look into that. There's also some good video recorders that can be found on the internet, free or paid. A list of them can be found in the Recording Video section of this page: LP guide for newbies - Let's Play Wiki
Or you can use a camcorder and hope no one notices.
The built-in recorders are best if you want a consistent frame rate for how the real game actually plays, if you want your video in real time with what you are actually seeing onscreen any recorder will do. Fraps is a popular free recorder for PC games.