There are for the moment many different menu applets. I would like to create a list of them here, so anyone can see which one fits best his preferences. So the goal is not to say one menu applet is better than another one. It is just that everyone know which choices exists.
Gnome Menu Bar
This is the default menu you get when installing ubuntu (and default in Gnome). It is divided it three parts: 'applications', 'places' and 'system'.
install: already installed and normally already attached
Gnome Main menu
This is a single logo. When you press on it you get the menu. Divided just like the Gnome Menu Bar. It takes only less place on the bar, but can do the same as Gnome Menu Bar.
install: already installed, but need to be attached to a panel yet. (right-click on a panel -> add to Panel -> Main menu (the one with the ubuntu logo, if there are multiple)
Slab is the codename for SUSE Linux Enterprise Desktop 10's new menu and application managment system. It consists of 3 parts: main-menu (the new menu applet), Control Center and Application Browser, a nice simple and easy to use interface which uses our existing menu system to present all options for preferences and applications right in front of you with nice big icons! The 2 sections are also searchable with this neat little search box, automatically narrowing down options to what you want based off title and descriptions!
status: beta for ubuntu
Ubuntu System Panel
Simple launcher for the GNOME desktop, providing easy access to Places, Applications and common configuration items for your computer. It has a plugin system and aims to be very personal.
status: 0.41 (early release)
Gimmie bases on the idea that people always have 3 things available when working on their desktops: Applications, Documents and People.
status: 0.2.5 (early release)
Affinty is a desktop search tool, which hopes to provide a quick way to get at all the different information on your desktop. It achieves this by having various back-ends, but implemented through one standard interface. Speaking of the interface, have a look:
status: 0.1 (first release)