To quote from here:

Custom table

You can create your own map and store it in your home directory (i.e. ~/.Xmodmap). Print the current keymap table into a configuration file:

xmodmap -pke > ~/.Xmodmap

Make the desired changes to ~/.Xmodmap and then test the new configuration with:

xmodmap ~/.Xmodmap

To activate your custom table when starting Xorg add the following:

~/.xinitrc
if [ -f $HOME/.Xmodmap ]; then
    /usr/bin/xmodmap $HOME/.Xmodmap
fi

Alternatively, edit the global startup script /etc/X11/xinit/xinitrc.

Test changes

You can also make temporary changes for the current session. For example:

xmodmap -e "keycode  46 = l L l L lstroke Lstroke lstroke"
xmodmap -e "keysym a = e E"

Also:

See man xmodmap for more details.

EDIT:

To clarify: the xmodmap ~/.Xmodmap may be used in many places, not just when starting Xorg. For example, I have these two functions in my ~/.bashrc:

   # map caps to esc

   mapcaps(){
      xmodmap -e "clear lock"
      xmodmap -e "keycode 0x42 = Escape"
      echo mapcaps: caps-lock set to Escape via xmodmap
   }
   unmapcaps(){
      xmodmap -e "keycode 0x42 = Caps_Lock"
      xmodmap -e "add lock = Caps_Lock"
      echo unmapcaps: caps-lock set to caps-lock via xmodmap
   }

This is so I can dynamically map Caps to Esc.

Really there is no limitation. Feel free to call xmodmap from ~/.xinitrc, ~/.bash_profile, from a custom script, etc.

The feature you are talking about is called "Auto-Scrolling". It lets you press and hold the middle mouse button and move your mouse to scroll smoothly. In Linux, the default behavior for this action (pressing middle mouse button) is generally pasting text.

However, there is a preference setting in Firefox and an extension available for Chrome/Chromium which would let you use the middle mouse button for scrolling and activate this feature.

Firefox

• Open the "Options" tab: "≡" (Open menu) → "Options".

• Navigate to "General" (it should open to "General" by default).

• Scroll down to "Browsing".  Under "Browsing", you will find the "Use autoscrolling" option. Put a check mark beside this to activate this functionality in Firefox.

  Or just search for "autoscrolling" using the search bar.

In older versions of Firefox: "Edit" → "Preferences" → "Advanced" → "General" → "Browsing" → "User autoscrolling".  Click on the below for a larger image.

    

Chrome/Chromium

For Chrome/Chromium we can use an Extension called "AutoScroll" (from kaescripts.blogspot.com).

• Go to this link on Chrome Web Store (obviously using Chrome/Chromium).

• Click on the button labeled "+ ADD TO CHROME" to install this extension.

  

• Click on "Add" in the Confirmation Dialog Box.

Other Applications

As far as other applications are concerned, I haven't yet found a solution for them. Anyways, it's the tall webpages that create most of the problems for which both Firefox and Chrome/Chromium have a solution.