Diskeeper Professional Premier is an application that prevents disk fragmentation. I also allows you to defragment disks and keep drives fast and in good condition.
While it's not really complicated, it appears so on startup. Unlike other defragmenting software, Diskeeper Professional Premier has a large, intimidating interface. However, performing first scans and defragmentations isn't at all difficult, and the process is well illustrated, so you can see what is being done.
While running, Diskeeper Professional Premier uses a system it calls 'IntelliWrite', which prevents files being fragmented as they are written to disk. This system seems to work well, and is surprisingly non-intrusive. The program will also run defrags in the background, without eating too much memory. This means you can defragment your drives while continuing to work, which is useful.
Diskeeper Professional Premier is a very complete defragmentation application. It's pretty big, and with its detailed reports, it probably does much more than even high-end home users require
If you're looking for a complete defragmentation solution, Diskeeper Professional Premier does everything you could hope for.