Windows: If you like your software open-source you'll definitely want to check out UltraDefrag, a snappy open-source Windows disk defragmenter.
UltraDefrag is available as a portable application and in a GUI and ultra lightweight command-line only version — the lightweight version is designated as "micro" in the download list and is at the top of the screen, make sure to scroll down for the GUI version if you want to interact with the program like in the screenshot above.
With UltraDefrag you can defragment your entire disk, defragment based on the file and folder — handy if your disk defragmentation is limited to a handful of frequently edited and altered files — and set it up to defragment during the boot process to access files Windows usually has locked.
UltraDefrag is open-source and available for 32-bit and 64-bit editions of Windows. Have a favourite tool for managing your disks? Let's hear about it in the comments.