I don't use disk defragmenting utilities. I find that in the real world they don't deliver the performance improvements they promise. On your server you will find that other bottlenecks will normally dramatically outweigh any (tiny) improvement in performance from defragmenting files.
That said, PerfectDisk or Diskeeper are the two big players in this market, and they both have products available for Windows Home Server. Their products are well-reviewed in general (though I don't recall recent reviews of their WHS offerings) so at least they shouldn't do any harm.
I'm not on the WHS team, I just post a lot. :)