Defragmentation Software
None of that actually. The most important thing is that it can take care of fragmentation in the most effective way possible. Not all of them are equally good at getting rid of fragmentation. Sometimes the differences might be minor and only then should you look for anything else of interest such as better user interface, faster defrag sessions, ability to schedule defragmentation or even complete automatic mode.
The best way to compare and rate any software is to see what the users are saying. Companies will often exaggerate and confuse their visitors into thinking that their product is actually the best, but if enough people are telling you otherwise, you should listen.
How to Search for Reviews
Use google. Type the name of the product and add "review" at the end of your query. Browse the pages and skip whatever might seem as crafted specifically to convince you to download or pay for the product. Websites such as www.productreview.com are often created by people affiliated with the company, so it is always a good idea to question the genuinity of such.
Instead, browse the websites that are obviously filled with everyday people and see what they say.
For a quick comparison of commercial defragmentation software, please see:
http://disk-defragmenter-software-review.toptenreviews.com/
Onto the free ones:
Auslogics Disk Defrag
License required for businesses, but free for home users. Supports scheduled defragmentation as well as automatic mode when your PC is idle. The interface is very user friendly and the application itself is fast and reliable. Defragmentation can be done within a matter of minutes depending on your hard drive size. Comprehensive reports are available. Highly recommended.
Official Website | Download

Piriform Defraggler
Definitely one of the best free defragmenters. Defraggler will equip you with all of the standard capabilities like scheduled defragmentation and custom filters, but also with an option to perform quick defragmentation, replace the standard disk defragmenter utility and even automatically update itself. The program is capable of showing you the hard drive's temperature and general health. Quite handy.
Official Website | Download

Disk Tune
A very lightweight defragmenter with a couple of really useful options. It can defrag, optimize and compact your disks. Optimization includes both defragmentation and re-arrangement of your files for better use. It also supports a thermal monitor and it will pause everything whenever your disk temperature gets too hot. If you prefer lightweight applications, this one is for you.
Official Website | Download


MyDefrag
Formerly known as JkDefrag, developed by Jeroen Kessels in 2004, this defragmenter offers a very clean and straightforward user interface with a tooltip for every option. Defragmentation is fast and easy and the program itself has a built in scripting language which allows users to perform or alter single actions supported by the application. Those can be anything from quick defragmentation to hard drive space consolidation.
Official Website | Download

UltraDefrag
Originally created by Dmitri Arkhangelski in 2007, UltraDefrag is capable of performing fast scans, filter customization and has the ability to automatically stop defragmentation when a specific time interval passes. It is translated into additional languages and can be run as a portable application. It doesn't have the prettiest user interface, but it does come with a lot of features that only top defragmenters have.
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