This mod adds an altimeter and speed indicator to the upper-left hand corner of the screen while piloting. Altitude is pretty accurate. (If the gauge does not correctly read zero when using a custom ship, there is a number inside the script you can change!) 

Current Version:
To install: Move the Altimeter and Speedometer folder to your Mods folder
To uninstall: Remove that folder from the Mods folder
This mod is intended to work on version 1.2.0 but might work for a while after that.
This version doesn't have a readme because I'm lazy.
1.1.7 version and older:
To install: Copy the EGMenuHUD.lua file to your scripts/gameplay folder.
To uninstall: Copy the EGMenuHUDbackup.lua file to your scripts/gameplay folder.
This mod is intended to work on version 1.1.7, but probably works on 1.1.8 too.
Readme included.

To make a gas giant compatible with this mod: simply set "IsJovian" to true in that planet's definition (it counts as false if it doesn't exist), and it will count the top of the atmosphere as altitude = 0.