Apart from its main purpose, what is bitcoin network used for?

Question

In the Ethereum Homestead docs there is a paragraph

But as bitcoin began attracting greater attention from developers and technologists, novel projects began to use the bitcoin network for purposes other than transfers of value tokens. Many of these took the form of “alt coins” - separate blockchains with cryptocurrencies of their own which improved on the original bitcoin protocol to add new features or capabilities.

Questions are:
1. What are the best examples of such 'other purposes' and projects, especially the ones that are not altcoins?
2. What are the most prominent altcoins that haven't lost their significance?

Answer 1

What are the best examples of such 'other purposes' and projects, especially the ones that are not altcoins?

Things that do proof of existence like OpenTimestamps are not for transfering money but rather committing something (in this case, a hash) to the blockchain so that it is essentially preserved forever.

Answer 2

As Andrew said, Timestamping is one powerful use of the blockchain. Another example is Namecoin which uses the blockchain for DNS registration. And also, coloured coins, which are used to keep track of ownership of basically anything being transferred around. The blockchain can also be used for storing certificates, check out Blockcerts. There are probably many more examples, people are very creative :)