There are several video sites on the internet like Vevo, Dailymotion, Vimeo and Viddler but none have the scale, the depth and the breadth of YouTube. This service started exclusively with user-generated (and of course pirated) content but has slowly evolved into a much larger video store. YouTube now has movies, TV shows and music (although the collection is not that deep yet) and since the last year they have started a little bit of live streaming as well.
Despite having a lot of competitors, it does seem like YouTube has the critical mass, the scale and a huge brand recognition, making it extremely difficult to be replaced.
Google Voice is a relatively new product which came from Google’s acquisition of Grand Central. It allows you to set up a telephone number and use it like a permanent email address. Several neat features come for free, like automatic forwarding to multiple phone numbers, quiet/Do Not Disturb mode, call screening, voice mail notifications and transcription and a ton of third-party mobile apps to work directly with Google Voice. There are competitors in the enterprise space (which are not free and cost a lot, in fact) but I have seen very little competition in the consumer space.
Update: Two more products which I forgot to mention here which were brought to my attention by Tom Reestman are Google Analytics and Feedburner. Both of these products are now utilities which have become a must for any website operation, and at the same time, have really no competitor of note. I use Google Analytics for my personal blog, and am actually extremely impressed with how much functionality they provide for free. I have stopped using Feedburner for quite some time now, relying instead on RSS (and corresponding statistics) provided by Posterous and WordPress.
There are several other Google products which a lot of people use, but I did not consider because I do not use those much. For example, Google Shopping, Google Checkout, etc.
Of late Google has been under a lot of fire with their real namespolicy enforcement especially after they launched Google+. A lot of people who used to have perfectly running Google accounts under pseudonyms, had their accounts disabled and/or suspended because they did not have their real name listed. The issue here was that it was not just the Google+ account that was impacted, it was every single Google service they used which was suspended. As a result there has been a lot of talk about diversifying your digital services so that you are not putting all your web apps eggs in one basket.
I am hoping this series has helped you in starting to think about alternatives to Google and if you have started along those lines already, to identify some specific replacements.