There are several window manager mimicking the look and feel of NeXtStep's window manager. In my opininon Window Maker does this job best. Some of its feature are:

• built-in icon dithering with support for 4bit and 8bit displays handling of window groups as a whole (iconify all windows of a given application into a single icon)
  
• application Dock (similar to AfterStep's Wharf) that you can configure using drag and drop
  
• workspace Dock (aka Clip) which is a workspace specific dock extender "pinnable" menus for launching applications
  
• menus that are automatically redefined when the configuration file is changed--no need to restart Window Maker to have the changes take effect multiple workspaces
  
• gradient rendered and pixmap textured window decorations
  
• built in theme support
  
• keyboard traversal of menus
  
• ability to name workspaces directly from the menu
  
• ability to change preferences, like colors, fonts etc, without restarting
  
• Comes with a utility to set your root window background
  
• a built-in GUI configuration utility.. no more hand editing files!
  

You can either install Window Maker as a port or as a package.

neXtaw is a modified version of the Athena Widgets. When you install it all your applications based on the Athena widgets will have the appearance of NeXtStep. Even without recompiling them!

• Install the libneXtaw - package
  
• become root and change to /usr/X11R6/lib
  
• move libXaw.so.6.1 to libXaw.so.6.1.old
  
• create a link: ln -s libneXtaw.so.6.1 libXaw.so.6.1
  
• create a file .Xdefaults with at least that contents.
  
  

• *Scrollbar.DrawBorder: must the black border around scrollbars be drawn?
  Example: XTerm*Scrollbar.DrawBorder: false
• *Scrollbar.DrawArrows: shall the arrows in the scrollbars be drawn?
  Example: XTerm*Scrollbar.DrawArrows: false
• *Scrollbar.AlwaysVisible: shall the arrow buttons and the slider be always visible?
  Example: XTerm*Scrollbar.AlwaysVisible: true
• *Toggle.ToggleStyle: Which appearance shall toggle widgets have?
  Example: xfontsel*Toggle.ToggleStyle: check # old/check/radio
• *MenuButton.MenuButtonStyle:: Which appearance shall menu buttons have?
  Example: ghostview*MenuButton.MenuButtonStyle: action # simple/action/select

To get a neXtish look for your GTK-based applications - for instance The Gimp - you can use the port x11-toolkits/gtkstep.

• Install the port
  
• Create the following link in your home directory:
  ln -s /usr/X11R6/share/themes/Step/gtk/gtkrc .gtkrc
• Now all gtk-based applications will have a neXtish look