Watch this page for upcoming games that are temporarily free on steam (some free to keep forever)
https://steamdb.info/upcoming/free/

Bundle of games:
https://www.humblebundle.com/
https://www.fanatical.com/en/bundle
https://www.indiegala.com/
http://www.dailyindiegame.com/
https://groupees.com/

Track the bundles:
http://v1.steam.hlxgame.cc/groups/IndieBundleTracker#announcements

With Enhanced Steam broweser extension games you've 'not interested' or 'wishlisted# and owned games are color highlighted. It also gives you better steam store search options and inventory options.
http://v1.steam.hlxgame.cc/groups/enhancedsteam

Search for the cheap games:
http://dailyindiegame.com/content_digaccount.html
https://www.kinguin.net/ (Kinguin net search for 'steam' and set max price to $€1)
http://store.steampowered.com/search/?sort_by=Price_ASC&category1=998%2C996&category2=29&filter=ut2
https://www.bundlestars.com/en/games?page=1&price=0,499&types=game,dlc&sort=discount
https://www.indiegala.com/store/search?type=games&filter=discounted
https://www.humblebundle.com/store/search/drm/steam/sort/discount

Steam discount coupons:
http://v1.steam.hlxgame.cc/groups/coupondumpster

Wishlist discount tracker:
https://isthereanydeal.com/

Cheap key finder
http://www.allkeyshop.com/blog/ (allkeyshop)

Game Giveaway Notifications*, too many groups to list, but here's a few:
http://v1.steam.hlxgame.cc/groups/IndieKings
http://v1.steam.hlxgame.cc/groups/Grabthegames
http://v1.steam.hlxgame.cc/groups/vantom
http://v1.steam.hlxgame.cc/groups/freegamesinfoo#announcements

*To be honest though I can't be bothered with these giveaways they require too much 'like this' 'do that' faff and gleam.io got my twitter account banned and no to the facebook stuff and I don't have accounts on some other sites etc.