Why don't ext2/3/4 need to be defragmented? Is there no fragmentation at all?
Modern filesystems, particularly those designed to be efficient in multi-user and/or multi-tasking use cases, do a good fairly job of not fragmenting data until filesystems become near to full (there is no exact figure for where the "near to full" mark is as it depends on how large the filesystem is, the distribution of file sizes and what your access patterns are - figures between 85% and 95% are commonly quoted) or the pattern of file creations and writes is unusual or the filesystem is very old so has seen a lot of "action". This includes ext2/3/4, reiser, btrfs, NTFS, ZFS, and others.
There is currently no kernel-/filesystem- level way to defragment ext3 or 4 at the moment (see http://en.wikipedia.org/wiki/Ext3#Defragmentation for a little more info) though ext4 is planned to soon gain online defragmentation.
There are user-land tools (such as http://vleu.net/shake/ and others listed in that wikipedia article) that try defragment individual files or sets of files by copying/rewriting them - if there is a large enough block of free space this generally results in the file being given a contiguous block. This in no way guarantees files are near each other though so if you run shake over a pair of large files you migth find is results in the two files being defragmented themselves but not anywhere near each other on the disk. In a multi-user filesystem the locality of files to each other isn't often important (it is certainly less important then fragmentation of the files themselves) as the drive heads are flipping all over the place to serve different user's needs anyway and this drowns out the latency bonus that locality of reference between otherwise unfragmented files would give but on a mostly single user system it can give measurable benefits.
If you have a filesystem that has become badly fragmented over time and currently has a fair amount of free space then running something like
For small amounts of fragmentation, just don't worry about it. I know people who spend more time sat watching defragmentation progress bars than they'll ever save (due to more efficient disk access) in several lifetimes of normal operation!
There is not much fragmentation as long as the disk isn't filled too much (so don't fill a disk up too much if you can avoid it).