If I had a billion dollars to spare, I would look around at all my new found friends I never knew I had plus I would be amazed how all of a sudden people would start to agree with me, thus proving my superior mental abilities and reinforcing my view of myself as a genius ;)

I would donate the money to a research institute developing efficient ways to harness solar energy. I would attach a mission to the money to make the company/institute invest the money in research to beam energy from space to Earth.

I would put half of it in the bank. The other half I would use on a spending spree.

I would place 800 million in the bank. 100 million I would use to play the stock market (FTSE 100 index of course, we'll bounce back!). 50 million to cancer research. 50 million to set up my own corporation specializing in hydrogen power.

I would waste half on escort girls and save the rest in a bank.

I would place 200 million into the bank, invest 50 million into the stock market, donate 500 million to research in areas such as HIV and cancer, the rest I would use to start a telecommunications corporation.

I'd create my castle. I made the floor plans some years back and it is about ten city blocks in diameter, which yes, means it is somewhat radial. Seven heptagonal corridors line the outermost 'ring' of the structure, what I call the public zone. The rest will not be private and will be known by only me and those whom helped me construct it, whom I will have killed. Don't feel bad though, their death will be predetermined in their contract.

I would fund a radical libertarian organization with most of it; I'd buy up most of the businesses in my city; I'd buy myself a large house and fill it with books and animals, spending my time amongst them both.

I would keep 500 million for my own purposes...

10 million to charity to shut everybody up

10 million to split up between parents and relatives

put 400 million in the bank

the last 80 million can be spent on whatever.

The government would take much of it, friends and family would beg at the dinner table, and by the time I would want to buy anything, the money would be gone.

I'd have several islands constructed in the Atlantic ocean. Those islands would be inhabited by ten thousand volunteers each. Every island and it's respective people would work under it's own pure system of government and economic policy. This would allow me to compare them in actual effectiveness.

...I would take a break, go on vacation, pay off everything, etc.

With a billion dollars, I could start that ice cream factory that I always dreamed about plus have plenty to spare to give to the homeless.