A lot of it depends on which Desktop Environment you are using. For example, there's Baghira for KDE 3.4 & 3.5; there are some tutorials out there on how to use Baghira for a somewhat Mac OS X-like look. You probably can do similar things with GNOME and Xfce. Even if you just do a Google search and include the terms "linux" and "Mac OS X" and "look like" you'll get some hits for tutorials and such. Without knowing which Desktop Environment or Window Manager you're using, I can't be more specific. Here's a link to one of the tutorials: http://baghira.sourceforge.net/OS_Clone-en.php
. One thing to note: people vary in their opinions about how much these themes, docks, etc. really make a Linux graphical environment look like OS X. Another thing: depending on how much you want your desktop to look like OS X, you may need to compile some things on your own; for some of the stuff out there I would imagine there are no Vector/Slackware packages. Some of the packages you might be able to find at http://www.linuxpackages.net
I hope this helps.