FILE MANAGERS
In Linux, you can replace the default file manager. I have replaced Nautilus, the default file manager in GNOME, with Thunar [link] Thunar is very fast and lean on memory usage (erm, well at least, that's the impression I get). I use PCFileMan [link] as well, which is a tabbed file manager.
DOCKS
Avant Window Navigator [link]
This is the first dock I used in Linux. It features the reflection of icons (similar to Leopard's dock), and various applets (stacks!). I recommend using this one.
Cairo Dock [link]
I recommend this one also as it features a nice parabolic zoom engine (if you like that). It's also very customizable, perhaps much more than AWN above.
THEMEING
Emerald [link]
It's a window decorator that has smoother borders than the Metacity windeco. Many of the window decorations I feature in my ss are Emerald themes.
Some gen. info: There are themeing engines for GTK which have a set of aesthetic elements (ie buttons, menus, scrollbars) that remain consistent when utilized; they may be customized for colour or look. One such engine is Clearlooks (found in GNOME as a default); its aesthetic elements are "gummy." Murrine [link] is another engine, and where Clearlooks is "gummy" it is more glassy.
THAT MAC MENU THING
Global Menu [link]
This feature is not found in a default GNOME. But I would recommend it IF YOU KNOW WHAT YOU ARE DOING. PLEASE READ THE INSTRUCTIONS CAREFULLY ON THE WIKI AND READ OTHER USERS' EXPERIENCES WITH GLOBAL MENU ON THE FORUM THREAD.