UltraDefrag 4.0.1 RC8 - an open source disk defragmenter for Windows NT 4.0 and later operating systems. UltraDefrag 4.0.1 RC8 is very fast, because the defragmenting is done by kernel-mode driver. UltraDefrag makes use of the same defragmenting API used by many other windows defrag utilities, both open and closed source. 

Changes in UltraDefrag 4.1.0 (2010-03-):
- few bugs in driver were fixed
- a huge analysis speedup performed
- one little bug preventing the Scheduler running on Windows 2000 was fixed
- volume status messages were improved in UltraDefrag GUI
- the report saving speedup was performed