[wiml]

The old Bussard ramjet concept was to capture these high velocity protons (with some kind of magnetic field), cause them to fuse, and use the fusion energy for propulsion.

There are a few engineering difficulties with the idea but it makes for some good SF stories...

[fyver]

The spaceship in Poul Anderson's Tau Zero uses a Bussard ramjet.

[bejd]

Fantastic book. I read it for the first time about a year ago but I still think about it once every week or two. Thought about it as soon as the "spaceship" started to accelerate towards light speed.

[zwaps]

Also all Stat Trek Federation ships

[MilStdJunkie]

Relativity effects will start to bite. The proton's going to stop being interested in your magnetic field[1], and you're approaching the velocity of the exhaust mass of the fusion reaction.

[1] The momentum vector just completely flattens almost any other physical characteristic. I'm not sure there's even enough time for nuclear fusion to take place.