How can I justify plain text in Linux?

I'm composing a plain text (.txt) document that has left-and-right justified paragraphs. I manually add spaces, breaking words with hyphens if necessary, until the lines are uniform length. To use an example from my document, a paragraph like this:

These phantasmic balls have some strange properties, some neat, some
interesting, and others just indicative of lazy design. They're almost all
caused by the same design flaw: The game treats them too much like regular
balls.

will end up looking like this:

These phantasmic balls have some  strange properties,
some neat,  some interesting, and others just indica-
tive of lazy design. They're almost all caused by the
same design flaw:  The game treats them too much like
regular balls.

How can I do this?

It's okay to add spaces (without breaking words over lines), but I need to control the target width of the paragraph (in # characters). The example above is justified to 53 characters per line, but line width in my actual document varies from 60 to 79.

It'd also be great if there was a text editor with such a feature.

I already tried:

• OpenOffice and TeX: They just use formatting to change how the text is displayed.
• nano's Justify command doesn't count because it doesn't actually make the lines uniform width.

_{I accepted the following answer because it was the one that worked best for me. I found the emacs fill commands the easiest to use among the answers, and I appreciate how it recognizes even non-whitespace indentation (for stuff like # and // comments). However, the other answers all fill their own niches and I recommend others visiting this question to look at them as well.}

You can do this in emacs using fill-paragraph or fill-region (fill docs).

You need to pass a numeric prefix argument. The default fill column is 70.

Select the text to fill and do M-3 M-x fill-region.

I set the fill column to 53 for your sample text and got:

These phantasmic balls  have some strange properties,
some   neat,  some   interesting,  and   others  just
indicative of lazy  design. They're almost all caused
by  the same design  flaw: The  game treats  them too
much like regular balls.

you need par (official website, source code, package status on Repology)

e.g. to get it on ubuntu, do:

sudo aptitude install par

to justify text.txt (at the width of 80 chars), saving in newtext.txt:

par j1w80 < text.txt > newtext.txt

to use par in vim or gvim:

:set formatprg=par\ j1w80

then highlight the text you want to format and use the gq command.

for more information, see man par