So, you're using a Unix desktop that's not A/UX or OS X yet you desire a certain level of Mac-like look-and-feel. This document is designed to help you find solutions to increase the Mac-ness of your desktop.
Suggestions for this FAQ are welcome!
The primary problem is that nearly every major toolkit used to build a window manager assumes menus-in-window. There doesn't seem to be a consistent API for apps to hand off menuing duties to a window/menu manager program, so if something like that is desired it has to be built independently—which means that only apps and window managers that share one of these toolkits can make use of this ability.
Shared (NeXT-like) menus are possible with GNUstep, and with WildMenus the menubar can be horizontal on top of the screen.
Étoilé (built on GNUstep) offers a Mac-like menu option, as well.
KDE gives you the ability to have a shared menubar at the top of the screen.
The following are not able to replicate the shared menu "feel", but do have Mac-like themes available to modify the "look".
The Macintosh Finder has traditionally used a spatial rather than a columnar approach to navigating the filesystem.
There are a number of free TrueType typefaces that resemble well-known "Macintosh" typefaces. Install them, and you can give additional "Mac-ness" to UI elements and your own documents.
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Tim Larson - FAQ list maintainer