Let's face it: you can sometimes forget a password, because well... it happens! I can't help you with all of them, but I can tell you how to recover your lost Linux root password thanks to Tips4Linux. Just follow the below steps to recover it:

So I have a laptop that I've been upgrading since Hardy (currently on Karmic Alpha) that I would like to boot faster. It has probably accumulated a lot of crufty daemons along the way that probably aren't being pre-loaded into memory. I picked up this tidbit from the fast boot expert. Add profile to your kernel command-line (at the grub prompt, press Esc e and then edit the line).

If your memory or mistyping leaves you without the right password to get into an account on a Linux computer, there's no need to reformat. Make Magazine's Hacks Blog gives a step-by-step account of the command line fix you can make by booting into "single user mode."

If You just installed Ubuntu and forgot what password you selected during the installation process or one of your friend gave you ubuntu computer without giving username and password.This tutorial will help you to recover username and password

Reseting Gnome Settings

If your gnome-panel corrupt , Application Menu, Places Menu and System Menu and some other menu items will disappear.
How to recover it

Boot the system upto Login Window. Press Ctrl + Alt+F1

Now You will get a Text based login screen

login with your username and password

now you will get a $ prompt.

Plenty of utilities can recover deleted files, but what if you can't boot your computer, or the whole drive has been formatted? Here's how to dig deep and recover the most elusive deleted files, or even whole partitions.

Forgetting passwords isn’t generally a good idea. It does happen, though, and if it happens to your MySQL server, this is the process you need to follow in order to recover it.

Password-protecting the bootloader is one method you may employ to enhance the physical security profile of your computer. GRUB, the GRand Unified Bootloader, is the default bootloader on virtually all Linux distributions, but on a significant number, the installer does not have support for setting a GRUB password.

Here's a scenario: you're locked out of plesk admin, you forgot the password and can't recover cause your email address is not set in the contact details.