Helium
Helium is a floating browser window that allows you to watch media, browse the
web and do much more while you stay productive. Your content will never fall
behind your other windows even as you switch tasks. Helium supports a
customizable translucency mode that allows you to simultaneously see your
content and your work, without intercepting mouse clicks when it's translucent.
You have the ability to click, drag, scroll behind your floating content.
Getting Started
Head over to http://heliumfloats.com and install the latest version of Helium.
If you don't already allow apps from unidentified developers, you'll need to
right-click and choose "Open" the first time you use the app (instead of
opening the app with a single click).