All read/write filesystems get fragmented, even Linux ones, although usually far less than NTFS. XFS is the only filesystem for Linux which has an online defrag utility - xfs_fsr
Ext2/Ext3 works in a rather different fashion to NTFS and FAT which tend to shove bits anywhere that is free on the hard disk. Ext3 always puts the file's bits as close together on a hard disk as possible, resulting in such dramatically lower fragmentation as to be negligible except on a system with an extremely high amount of file creation and deletion, e.g. an enterprise level or above mail server. Even then the impact on performance is minor at best, unless you're operating with only a small amount of disk space free; unlike, say, NTFS where even a monthly defrag will see huge benefits. Off the top of my head there is an ext2 defragger, but it's not been updated to work with ext3, or if it has no one has found significant enough advantage in it to bother releasing it. With the way Linux development takes place if there was any benefit at all of defragging it there would be a mainstream open source utility for it. There is one on its way for ext4, but performance advantages from it are shown to be so minor it's almost not worth wasting time with.
File systems like ReiserFS4 automatically defragment as you go along, ResierFS doesn't and isn't quite so good about fragmentation, but hey that wife murderer's file system isn't so mainstream these days as to be a problem
XFS uses something called lazy evaluation for allocating files. Instead of being writted straight to disk a file is written to the buffer cache and instead of allocating specific sectors for the data, XFS reserves the appropriate amount of space for the data held in the buffer cache. Specific block allocation only happens when XFS comes to finally write the data to the actual hard disk. This pretty much dramatically reduces the chances of files ending up fragmented on a hard disk, along with associated performance advantages. When I've been administrating mail servers for ISP/Enterprise level, XFS has been my preferred file system for the heavy accessed mount point, primarily because the large number of reads and writes are transitory.
If you're really really bored, utterly convinced your filesystem is fragmented, and have good backups you might try this script, but no promises:
http://ck.kolivas.org/apps/defrag/
A tool for finding how fragmented your ext3 filesystem is:
http://sourceforge.net/project/showfiles.php?group_id=132340
