Steam offer various methods to help friends and family purchase games, with the ability to gift games and purchase physical gift cards from retailers. Steam have introduced another method to help top up your friends and family's Steam Wallets, with the introduction of Steam Digital Gift Cards.
As you could imagine, it's pretty straight forward and works much in the way of physical Steam Gift Cards... except you don't have to leave your house! Users sign into their Steam account, select the amount you wish to gift and the recipient and then the funds will then be transferred straight to the recipients Steam Wallet. These funds can then be spent as they desire- hardware, software, games or in-game items.
If you're feeling generous, head over to Steam's website for more details.