Nyanit.com is a website where inserting any website's URL into the bar on the main page will make Nyan Cats spawn all over the chosen website along with the Nyan Song playing constantly in a loop.
Type the website you want to put nyan cats on into the bar. Once that is done, click the "nyan" button and it will load the page, and the nyan song will play, along with "OMG!!!!!!!!!!!!!!!". Clicking the "Nyan Me" button and then once the selected page loads, nyan cats will spawn.
Clicking on the cats makes more spawn, whilst leaving them alone makes them grow over time, until the whole page is invaded by nyan cats.
This website is good for making "Nyan Cat Invasion" videos.