Steam Trade Matcher [www.steamtradematcher.com]: Trade your duplicate cards.
Steam gifts [www.steamgifts.com]: Win free games.
Reddit's Game Deals sub : Good place to find game deals.

Stores that sell bundled games, official resellers
Fanatical [www.fanatical.com]:releases bundles 2-3 days a week, each day can offer multiple bundles.
Humble Bundle [www.humblebundle.com]: Monthly subsciption and also releases 1-2 other game bundles a month.
Indie Gala [www.indiegala.com]: Indie game bundles. 1-2 bundles a week.

Software
Archi Steam Farm [github.com]: Trade card farmer. 100% safe.

Browser Plugins
Augmented Steam [es.isthereanydeal.com]: Steam store enhancements.
Tampermonkey [www.tampermonkey.net]: Allows to run scripts.

Scripts
Steam Web Integration [github.com]: Check every web page for game, dlc and package links to the steam store and mark if it's owned, unowned, ignored (not interested), removed/delisted (decommissioned), wishlisted or has cards using icons.
SteamTrade Matcher Userscript [www.steamtradematcher.com]: Script to help with trading cards at Steam Trade Matcher.