Note: mintMenu is based on a fork of USP (Ubuntu System Panel), but the two are not the same thing, mintMenu being a lot nicer.
Here is how Gnome Main Menu looks like:
And here is the Linux Mint menu:
I also like the customization Linux Mint menu offers (right click the menu and then select Preferences): you can choose to display or not to display the recently used items (while those are always displayed in the Gnome Main Menu) but not only that; you can also toggle on/off the side pane, chose the icon size, colors, number of favourite items and so on. Basically everything is customizable.
You can also click the header of a section to make it go away, and have just a button for it on the left pane:
Of course, you can also change the Linux Mint icon and use an Ubuntu logo instead of it and even set the key to bring down the menu, directly from within the Linux Mint menu preferences:
Convinced? Here is how to install this great menu in Ubuntu.
Install Linux Mint (USP) menu in Ubuntu
To install the Linux Mint menu in Ubuntu, all you have to do is paste this in a terminal to download and install it:
wget http://packages.linuxmint.com/pool/main/m/mint-translations/mint-translations_2010.02.02_all.deb
wget http://packages.linuxmint.com/pool/main/m/mint-common/mint-common_1.0.5_all.deb
wget http://packages.linuxmint.com/pool/main/m/mintmenu/mintmenu_4.9.9_all.deb
sudo dpkg -i *.deb
sudo apt-get install -f
Then, to add the Linux Mint menu to a Gnome panel, right click a panel and select "Add to panel", then search for "mintMenu" and add it.
Update: You can now install Linux Mint Main Menu (MintMenu) From an Ubuntu PPA