Imperium is freeware, but not open source. It is available for Windows and Linux, but doesn't come in a portable version. The file manager looks great and works well for the most part, and even though it is mouse-friendly, there are a ton of keyboard shortcuts. The program is a bit slow to start and appears to lag slightly, which I think is because of the animations. The program's resource usage was around 115-125MB of memory and about 5% of CPU, though it jumped up to about 10% for a second or two, when switching to a different drive or a folder with hundreds of files.