Medkit

In the Half-Life series, Medkit (or Health Kit) is a healing item. Medkits are usually placed in high-risk areas where immediate medical attention might be needed.

Half-Life 1
In Half-Life 1, the amount of health Medkits recover depends on the skill level. In Easy and Medium, Medkits will recover 15 health, while in Hard, Medkits will heal 10 health. The small amount of health Medkits recovers means that you'll usually be using Chargers to recover a lot of lost HP, especially in Hard mode.

Half-Life 2
In Half-Life 2, Medkits will always heal 25 health, irregardless of the difficulty setting. This makes them the main source of healing throughout the game, as they're much more common than Health Chargers.

Of note is that the Medkit in the Beta looks exactly like the one in Half-Life 1, only without a strap.

Other
In Team Fortress Classic, the Medic's primary weapon is a reusable Medkit. It will heal an ally to 100 health if they are under 100. If the ally is over 100, it will boost them up to 150% of the target's max HP. If it is used on an enemy, it will infect him with a deadly disease that can only be cured by a Medic.

In addition, Medkits can also be found on the ground in TFC, most notably in 2Fort. These Medkits heal 25 health.