“The Steam Winter Sale 2018 is on; gamers from all over the world can browse thousands of discounted games on Steam and win digital items throughout the sale.”

https://www.ghacks.net/2018/12/21/the-steam-winter-sale-2018-has-started/

The sale is running till January 3, 2019.

Valve added a winter event to the sale, you can open a door of the “extremely cozy cottage of surprises” each days to receive digital item like emoticons, wallpapers or items and knick-knack. They can be sold on store for some cents and the knick knack items can be consumed to level up a badge.

You get 3 Winter sale 2018 cards if you explore the queue of game suggestion, they can be combined into badges or sold on the market.

The usual Steam Winter Sales rules apply:

Games may be offered for cheaper on other platforms, e.g. gog which holds a Winter Sale of its own right now. Also, games on gog are DRM free.
You can buy any game at any time as it won’t be offered cheaper during the sale.

Virtual items that you gain throughout the sale may be sold on the market; you may make a couple of bucks this way to buy a game for free.
Many DLC for games are discounted as well; sales are a great way to buy DLC, even all of them, through collections.

If you are unsure about a game, add it to your wishlist on Steam and wait for another sale: there are plenty of sales throughout the year.
Here is a personal selection of games that are under €10 that you may want to check out:

Gabriel Knight: Sins of the Father 20th Anniversary Edition for €4.99 — Remake of an excellent adventure game.

Hotline Miami for €2.12 — Adult game, lots of violence in it.
Orcs Must Die Complete Pack for €3.24 — Defend against onrushing hordes of monsters. Build traps to assist you.

Portal 2 for €1.63 — Classic puzzle game by Valve. Excellent, fun, especially if you play it with a friend.

Tabletop Simulator for €9.99 — If you like boardgames, this is the game for you. Play hundreds of boardgames with other enthusiasts online.

The Talos Principle for €5.99 — Great game that has received rave reviews since its release.