Web or browser push notifications are messages from a website you receive in your browser, regardless of the device may it be a smartphone, laptop, or desktop. They notify about promos, important updates, or new content. It’s a new marketing channel and a good supplement/alternative to email marketing. It is in fact easier as people do not need to give away their email addresses to subscribe.
TruePush and FoxPush are 100% free unlimited web push notification providers. Aside from the free plan DigiTalPush also offers a monetized plan with the option to limit the number of ads. ProPush is also monetized but they are spammy.
PushWizard caters to iOS and Android apps.
OneSignal gives you unlimited mobile push but web push is limited to 30,000 subscribers only. It has a WordPress plugin that can send a notification when a post is published. WebPusher is free for 60,000 subscribers and below.