There’s one that I like most: Double Commander. It’s totally free, no shareware or demoware, and open source and it’s created using Lazarus+FreePascal! Awesome!
If you looking for an alternative to a shareware Total Commander, you are choosing the right one. Double Commander can also install+load plugin from Total Commander.
Although I said in the title it’s “for Windows” but Double Commander can run on Linux too. Since it’s created in FreePascal, it should be able to be compiled on Mac also. However the only available binary is only for Windows (32 and 64 bit) and Linux (also 32 and 64 bit).
If you don’t want to bloat your Windows registry, Double Commander also come in “portable” version. No need to install, just extract and you are good to run it.
Some notable features:
If you want to download more recent (bleeding-edge/alpha) Double Commander for Windows 32bit and 64bit, you can head to: http://www.paehl.com/open_source/?Double_Commander