If I took off in a space ship and went in a straight line, where would I end up

Well, its just the concept of endless space I have never been able to come to terms with

This has to do with the shape of the universe and there still isn't very many guidelines on what would happen.

If you look at a game of asteroids where if you zip off the right side of the screen, you appear on the right side and if you zip off the top of the screen, you appear on the bottom; the shape of that game is the two dimensional surface of a torus.

One of the analysis of the fundamental frequencies in the CMBR map indicates that the universe may be a dodecahedron. With a dodecahedron, not only would you appear on the other side of the universe but you would be rotated. However confirmation of this has yet to be found.

The series "Through the Wormhole" did an episode on this.

Far, far away. I cannot tell you where you'd end up because you didn't indicate which direction your straight line heads.

We know we can see stuff billions of light years away -- not a time comment but a distance one -- one light year is 6 trillion (million million) miles, and we're talking about thousands of millions times that!

Endless? We've not found an end, but there could be one out there.

"The View from the Center of the Universe" is a book from 5 or 10 years ago that might help with the concepts.

It would totally depend on where you would take off, and in what direction you are taking off, it would depend on the fuel source and how long it would last, it would depend on your life span and the amount of amenities you have created for yourself such as food, medicine etc. The Universe is a vast and great space filled with beautiful things, yet mostly it consists of a vast vacuum of nothing, but with the technology that exists today you would probably reach a distance of 120 astronomical units (1.8x1010 km) just like Voyager 1 did as of February 2012, after 34 years, it is the farthest man made object from Earth.