I like Defraggler, as it:
Downsides? None that I can see of. Perhaps it could detect that the Windows Disk Defragmenter is running in the background, and warn you that running two disk defragmenters at the same time is going to slow things down.
You can download Defraggler here.
Back in February 2008, I wrote about what disk fragmenting was. A reader then followed up with
“Well, what are my choices for a Windows disk defragmenter?”
Here is a list of some of the choices out there, if you don’t want to use the built-in Windows defragger.
Free
| Name | Recommended | Reviewed | Comment |
| Auslogics Disk Defrag | Yes | 20 May 2009 | |
| Defraggler | Yes | 11 March 2010 | |
| JkDefrag | Yes | A graphic front-end is available (JGDefragGUI) | |
| MyDefrag | |||
| Smart Defrag |
Not free
| Name | Recommended | Reviewed | Comment |
| Diskeeper | Yes | No | Been around for years. I first used it with Windows NT 4. |
| DiskTrix UltimateDefrag | |||
| O&O Defrag Pro 11 | |||
| PerfectDisk | Yes | 8 February 2008 | I have used this on work computers, and I’m very happy with it. |
| Puran Defrag | |||
| Vopt | Yes | Very popular with the folks over at Whirlpool. |
Some screenshots
Select which drive you want, note the absence of Removable drives. 
I clicked next, and away we went with defragging 
And we’re done: 
Auslogics Disk Defrag can be found here.