Anyone knows how to hide or dim the menu bar ?

I don't know of a way to hide it but you can dim it somewhat by making it translucent in System Preferences > Desktop & screen saver > Desktop tab > check "Translucent menu bar" at the bottom.

Regards,
Captfred

MagicMenu, but it may no longer work.

Auto-hide the dock and menubar on a per-app basis .

Hide Your Mac Menu Bar and Dock for a Cleaner Desktop.

All found doing a Google search. Why not give that a try first before posting here.

since the menubar is the central point to work with applications and get notified about things in your background applications there is no way to do that. and honestly, it's like 40 pixels or something? 😀

Well, then, I can only suggest you continue checking out links provided by your Google search, but I can't tell you if there is a solution.

http://www.blacktree.com/
Click on Nocturne and download the 2.0 version.

It does not make the menu bar "real estate" available. Nocturne is for people with tired or overly light-sensitive vision; it dims everything. It also has settings to invert or dim the menu bar even when Nocturne is off. You can set the dim effect to 0% to hide the menu bar. It's free and it's easy to use. I've been using Nocturne since OS 10.4 without any problems with it.