Would agree Google Adsense is the giant but they still have competition. A quick search on duckduckgo revealed at least some to investigate.

Note - many of these are probably just as scummy or moreso than adsense but they, at a minimum, are not part of Google.

    Media.net
    PropellerAds
    Amazon Native Shopping Ads
    Adversal
    Sovrn //Commerce (Formerly VigLink)
    Skimlinks
    Monumetric
    InfoLinks
    ylliX
    Evadav
    PopCash
    PopAds
    RevContent
    Adsterra
    SHE Media
    AdRecover
    MadAds Media
    Bidvertiser
    Adbuff
    BuySellAds
    AdClickMedia