Pennywise is a multiplatform desktop application that allows you to open anything in a small floating window that always stays on top of the other applications all the time, allowing you to multitask with ease. No need to keep struggling with alt + tab, use pennywise and have your work in front of you all the time.
Aurora is a floating browser to keep you entertained and productive.
FloatPlayer is a simple Mac app that plays a floating YouTube-video on your screen. The player is visible even on top of full-screen apps.
It's also completely open source.
Check it out on GitHub.