Of course, this is just a small selection of software that is now available for OS X...

Freeware

Carbon Copy Cloner
CCC will copy your currently installed MacOS X system onto another disk or partition, and will make it bootable.
http://software.bombich.com/ccc.html

Launchbar!
OS X GUI enhancement program.
http://obdev.at/

XPostFacto
Allows 604 processor Macs to run MacOS X 10.1.x, even ones that have processor upgrades.
http://eshop.macsales.com/OSXCenter/XPostFacto/

MacJanitor
Trims down log files and other files that get rather large, plus other house keeping tasks.
http://personalpages.tds.net/~brian_hill/macjanitor.html

DeLocalizer
Removes all unnecessary language files from MacOS X, therefore reclaiming a substantial amount of disk space.
www.bombich.com/software/local.html

Shareware

ASM
ASM adds an application menu on the Finder’s menu bar. Version 2.1x onwards is now shareware.
www.vercruesse.de

Drag Thing
Adds an application menu on the Finder’s menu bar, amongst many other customisable things it does to the Finder.
www.dragthing.com

Default Folder X
Folder and disk navigation enhancement for Open and Save windows.
www.panic.com

X-Assist
Adds an application menu on the Finder’s menu bar, amongst other useful things.
http://members.ozemail.com.au/~pli/x-assist/

Visage
Customise the log-in window and background, boot screen, boot strings, desktop effects and personal alert messages.
www.stanford.edu/~keaka/visage.htm