Even Facebook has its own personal assistant called M on the way, although it's not quite as comprehensive or as ubiquitous as the others quite yet. Here's how helpful all of the main digital assistant apps are right now.
While these apps all cover the same kind of features, they take a variety of approaches and have different priorities. Google Now is the one most like a real life PA: keeping on top of everything you're doing and everywhere you're going and surfacing important information before you know you actually need it.
For example, if Google Now knows you have a flight today, it will tell you the travel time to the airport and warn you if the flight's been delayed. Of course, this depends on it having access to your Gmail, Google Calendar and so on.
On top of that very personalised service you can also get answers to web searches or map queries (like "when was the Queen born?" or "where's the nearest coffee shop?") and it's possible to control a few smartphone features as well (you can turn Bluetooth on or off with your voice, for example). On a more philosophical level, Google Now points towards the future of smartphones - not a uniform grid of app icons but the right app at the top of the pile whenever you need it.
Siri is positioned and marketed as a very personal app, but compared with Google Now it spends more of its time in the background. In the past, Siri has been less keen than Google Now on trying to predict your needs based on previous behaviour, but with the iOS 9 "proactive" update that distinction is now less clear.
Siri makes more of its voice control features than Google Now: you can find phone numbers, launch apps, display photos, send emails, even post updates to Twitter and Facebook. Integration with apps (like Clock and Calendar) goes deeper with Siri than it does with the other personal assistants here.
As is the case across all these apps, it's possible to run web searches, set reminders and check on the weather forecast for the next few days. One important point to bear in mind is that Siri is only available on iOS devices, whereas Google Now and Cortana are spreading out to iOS, Android and the desktop.
When Microsoft first unveiled its Cortana app, it was described as taking the best bits from Google Now (advanced personalisation) and Siri (comprehensive voice control) and melding them together. Since then the distinction has become less clear, as these digital assistants continue to borrow features from each other, but it's still a helpful shorthand way of thinking about what Cortana is.
Like Google Now, it tries to know as much about you as possible; like Siri you can control various functions on your devices. With the arrival of Windows 10, Cortana is the most well-established personal assistant on desktops and laptops at the moment, but it's not quite as intuitive on smartphones (we're still waiting for Windows 10 Mobile) and it hasn't yet been fully launched in the UK.
Web searches, reminders, weather forecasts, personalised news... Cortana can handle all this with aplomb. Where it falls behind - especially considering the recent Google Now on Tap and proactive Siri updates - is in its integration with third-party apps, part of a bigger problem that has been dogging Windows Phone for many years.
Amazon Alexa is the outsider in the race, because it's only available on the Amazon Echo and the new Fire TV. However, Amazon clearly has big plans for its digital assistant - don't be surprised to see Alexa apps appearing on Android and iOS in the future.
Alexa can't dive into your emails or travel plans like Google Now, Siri or Cortana can - Amazon just doesn't have access to that information - but it can run web searches, read out audiobooks, check up on the weather and of course order stuff from Amazon. One of the most recent updates added support for various bits of smart home kit.
It has no visible interface, as everything works via voice control, but Alexa is actually very good at recognising natural language and converting it into actions. It does work with calendars, via the accompanying app, and we'd expect plenty more features (and devices) to be added to the Alexa ecosystem in the near future.