I've seen one that was a firefox extension, and plenty to download, but is there a website that I can visit to just play old NES games? Online, no downloads, etc.
I'm not looking for a directory site like addictinggames.com, or a site with a bunch of knock offs, but somewhere to play the original games in 8 bit form.
I found the following from Google, but I can't vouch for them as I never had a NES myself.
You'll need to have JAVA installed, but this should get you going: VirtualNES
Here's a pretty cool NES emulator: JSNES and it doesn't require any browser plugin, HTML5-based, it runs faster in very recent browsers.