Gmail, which another poster described, was also mind blowing at the time.
Google Maps, and the fact that you could scroll across a map smoothly was mind blowing at the time, and really redefined what was possible with xml http request, and "web 2.0"
These are the biggest that come to mind and its somewhat telling that Google Maps was the latest, and I believe launched in 2005 or 2006.
There were lots of other products that were less impressive, but definitely a step ahed of just about everyone out there- google finance, the "office" suite- do kids in school even install MS Office anymore? It used to be a defacto must have piece of software, I haven't had it on a personal machine in years now...
And a lot of stuff that looked like it was going to be something very big, but died on the vine. Google Voice, Google Reader, Google Image search come to mind here.
Then from a pure tech side of things, Go, Kubernetes, Node.js, the early hadoop and "big data" tools, all came from Google as well. But maybe getting back to your point, their backend stuff has not really turned into an "astonishing" user facing product in quite some time.
Being able to spin a globe, find our house, visit any place in the world.
Maybe it was less useful compare to maps, but definitely more fun.
Also Google labs, I remember playing with their set tool all the time. You add a few items of a set, and it added more. Ie 4-5 grunge bands and it will spit out 10 more.
I will say though more recently, when I first got in a car with Android Auto I was like I will never be able to live without this again. It may sound silly but just the fact that the time synced with my phone I was like take my money. Even more recently the call screen feature on the pixel 6 and 7 was just pure surprise and delight. I finally feel like I am the boss of my phone again. Google lens and translate are actually quite impressive but kind of niche use cases.
There were a lot of little things, Google desktop which was pretty cool for awhile and it kind of fulfilled what MS was trying to do with it's "Active Desktop."
My point being that maybe everyone (including myself) is a little harsh but google is still building interesting things. The pace seems to have slowed down though and they aren't leaps ahead of the pack anymore.
Chrome launching was also a pretty big deal, I recall.
But Chrome was just a re-hash of firefox, nothing exciting there.
But there's also a reason software people switched to Chrome for a long while. It was just better. It's not anymore. I switched back to Firefox when Chrome broke ad blocking.
I am a Firefox user now, and I was a Firefox user when Chrome came out, but Chrome was miles ahead of Firefox in pretty much most things that mattered for most people, especially performance and usability.
Chrome was a revolution for the web.
Google search probably is a better service than it originally was, but the combination of SEO and the rapid increase in complete garbage online probably explains why searching disappointing so much of the time now. Sadly they've also removed a lot of powerful features such as operators, and integrated calculations that were similar to Wolfram Alpha.
That was exciting
Wave was a good tech demo, but only alter they figured out what the tech can be used for.