Wow, seems like lots of unnessisary paranoia around here for 3rd party defrag tools. I've used a varity of them over the years and never had an issue.
Are 3rd party defrag tools ever nessisary? Of course not. Are they inherintly harmful to your PC? Also, of course not. If all you want is basic defragging, just use the built in Windows utility. But if you want some extra features, there are plenty of repuatable applications out there.
Defraggler and AusLogics Disk Defrag are my favorites.
Defraggler is very simple with a nice, clean interface. One of the reasons I like using 3rd party defrag utilities is for a graphical map of fragmentation (since MS decided to take that out after XP), and Defraggler has lot of customizations for that. You can also do boot-time defrag with it, which is really nice if you don't want to bother booting into a PE environment to get a 100% defrag. You can also set exclusions based on extention and file size, something else the Windows defrag can't do.
Auslogics can do most of what Defraggler does with an optional Optimize option, which has a couple practical functions. It can consolidate free space, which I believe Windows defrag does by default as well. It can inteligently move system files to the faster section of the hard disk (closer to the center of the platter) and consolidates them away from bulky data files. It can also prevent MFT fragmentation. All of the optimizations probably don't add up to too much of a real-world performance increase though.