One alternative is: 'To whom it may concern.' This sounds a little formal, I know, but it's very common when you don't know who specifically to address it to! :)

WRITING PRACTICE

As part of a class assignment you have to write about the following topic:

We are becoming increasingly dependent on computers. They are used in business, hospitals, crime detection and even to fly planes. What things will they be used for in the future? Is this dependence on computers a good thing or should we be more suspicious of their benefits?

A few years ago computers were created to make easier the daily life of human beings easier. In fact the first computers were designed to do complex numerical calculations that would take hours, days and even a week for a group of people to do. So that´s why this these amazing machines were have evolveding until today, and are still evolving. and they still are doing it. 

Nowadays computers are in every aspect of our lives. Their evolution of them is happening so fast that in the future they will probably displace humans from important and risky tasks, so important and risky like operating planes, ships, trains, and  cars, with almost zero intervention to eliminate accidents and deaths in every way area where transportation is used. Computers are being used especially in the field of medicine, The field of medicine is a significant point where computers are aimed, reducing the mortality rate of certain procedures and the treatment of extremely dangerous diseases extremely dangerous that today doesn´t don't have a cure or their have treatments that are painful and long.

Another thing area where computers could be used is the improvement and increase of human capabilities, making us stronger, faster, and more resistance to different environments, modifying our DNA structure, so that maybe we could travel long distances through space and live in other planets. This is a world of possibilities where the sky is imagination is the limit. (note: it's fine to say 'the imagination is the limit,' but 'the sky is the limit' is a very common colloquial phrase that native speakers use much more often to convey the same idea!)

All the fields where computers are used and are going to be used are certainly an improvement for human kind. Nevertheless, all that could be a problem if we don´t create a kind of insurance, because scientists now fear is that computers will develop a sort of artificial intelligence and displace us from mundane tasks of our daily lives.

All three of these technically mean the same thing, but 'I'm sorry to come over unannounced' is the most natural-sounding :) 'I'm sorry to come over without advance notice' actually can just be said as 'I'm sorry to come without notice!' (because notice means that it is given in advance; so you don't also need to say advance!) 'I'm sorry to come over without announcing myself' sounds very formal. I hope this helps! :)