First of all: There is NO REGISTRY in OS X, so don't even worry about that for half of a nanosecond. Secondly, Fragmentation in OS X doesn't really occur in the same way as it does in Windows. Check out this article for a pretty simple run down on the process and why using a defrag app is still useful: (read all the way through and don't skim)
Disk Fragmentation & OS X: When Does it Become a Problem?: Apple News, Tips and Reviews «
Onyx is a gem of a tool. Useful for several different things, and is a one stop shop for maintenance. (doesn't defragment though)
I've had my MacBook Pro for a bit over 3 years, and all I have ever done is run Onyx, keep all my photos and music on an external hard drive (keeps lots of disk space free) and once a month calibrate the battery. I have to do this though, since it's always plugged in as I use an external monitor for photo editing.
I'll admit, when I upgraded to Snow Leopard I did a fresh install and wiped the previous data with the ol' 7 x wipe. (writes over the data 7 times) Total overkill, but I'm anal like that.
Everybody here has given the same good advice. You really don't need to go overboard with maintenance as you would with Windows. Try keeping your home folder and download folders clean, don't store unnecessary data etc.. Sorry, I'm being a lot less detailed than usual. I'm really sleepy and am in fact... working on fixing my cousin's Windows laptop this very moment! haha.
Doug