Ubuntu Grub - Change Default OS

If you install Ubuntu as a second Operating System alongside Windows, every time computer boots you will be presented with a Grub boot menu where you can make a choose which OS to boot. If you don't make any choice in 10 seconds your default OS will boot. Obviously after installing Ubuntu it will become the default Operating System.
You may want to change your default OS or reduce Grub timeout. Probably easiest way to achieve this is to install StartUp-Manager.

Open Terminal and run sudo apt-get install startupmanager and wait until installation is completed.
Open Dash Home, type "Startup" in the search box and open StartUp-Manager
In StartUp-Manager select your Default OS and reduce reduce timeout if required. Click Close to save changes

Disable Ubuntu Unity Launcher Autohide

Unity 3D on Ubuntu 11.04 (Natty Narwhal) and Ubuntu 11.10 (Oneiric Ocelot)

Use this method to disable Unity Launcher autohide on Ubuntu 11.04. On Ubuntu 11.10 this will only work if you use Unity 3D. For Unity 2D check the second method below. Unity 2D was introduced in Ubuntu 11.10 to provide Unity interface for computers without accelerated 3D graphics. You can read more about differences here.

1. Install CompizConfig Settings Manager: Open Terminal and run: sudo apt-get install compizconfig-settings-manage

2. Open Dash Home, type "compiz" in the search box and open CompizConfig Settings Manager

3. Open Ubuntu Unity Plugin
Disable Ubuntu Unity Launcher Autohide

4. In Hide Launcher select Never.

This will affect only logged on user.

Unity 2D on Ubuntu 11.10 (Oneiric Ocelot)

1. Install dconf-tool tool: Open Terminal and run: sudo apt-get install dconf-tools
Disable Ubuntu Unity Launcher Autohide

2. To disable Unity Launcher auto hide, run: dconf write /com/canonical/unity-2d/launcher/use-strut true

3. To re-enable Unity Launcher auto hide, run: dconf write /com/canonical/unity-2d/launcher/use-strut false

Ubuntu - Disable Music File Preview on Mouseover

In Ubuntu by default if you click on a music file or just mouseover it will automatically start playing the file. Even if there is another file playing.

To disable this behaviour:
Nautilus (File Explorer) > Edit > Preferences > Preview > Preview Sound Files - Never

Ubuntu 10.10

Ubuntu - disable auto screen lock after suspend and hibernate

Disable auto screen lock after suspend and hibernate

Even if during installation you chose "Login in automatically" Ubuntu will prompt you for a password after starting from suspend (sleep) and hibernate.

To change this behaviour try this:

Option 1

System > Preferences > Screensaver
Disable "Lock screen when screensaver is active"

If this doesn't work try:

Option 2

Open terminal (Applications > Accessories > Terminal) and type gconf-editor
navigate to apps > gnome-power-manager > lock and disable all items
If this still doesn't work repeat this but launch gconf-editor with sudo gconf-editor

Even after this Ubuntu may still prompt for a password if you suspend or hibernate using Ubuntu soft power button (top right on the desktop). If you close laptop lid or use CTRL+ALT+DELETE dialog everything works as expected. This is a bug and is discussed in more detail here.

Ubuntu Desktop 10.10
Ubuntu Desktop 11.04