You can install Tampermonkey to do this.
For the newest stable version though, download the Download Chrome Extension extension and install the Chrome one.
Note though that it uses the Greasemonkey-type headers and auto-executes your code when the DOM is ready. As in, you don't do the window.opera stuff and use addEventListener("DOMContentLoaded"... like you would do with Opera 12 user scripts. Also, the script code is managed inside the browser in the extension settings instead of files/directories. But, it's the same thing basically.