Bundlestars.com and indiegala.com are the other two main game bundle sites I know of, but there are some others. Humble and Bundlestars are the two big ones, AFAIK, at least in terms of quality of games. Indiegala tends to bundle more but lesser reviewed, cheaper games ("bundle fodder"), in part because they post a new bundle practically every day.
Sites like isthereanydeal.com can notify you of sales and bundles for games you specify. ITAD keeps track of bundles (click on "Specials" on the main page for a list), too, and reddit.com/r/gamedeals will keep you up to speed on bundles and sales.
FYI, Steam has both Collections and Bundles. If you own a game in a collection, you don't get any discount (I have no idea if you get a spare key or what). If you own a game in a bundle, they lower the bundle price proportionally.
Sites like isthereanydeal.com can notify you of sales and bundles for games you specify. ITAD keeps track of bundles (click on "Specials" on the main page for a list), too, and reddit.com/r/gamedeals will keep you up to speed on bundles and sales.
FYI, Steam has both Collections and Bundles. If you own a game in a collection, you don't get any discount (I have no idea if you get a spare key or what). If you own a game in a bundle, they lower the bundle price proportionally.