You can try the cases env in amsmath.

documentclass{article}

usepackage{amsmath}

begin{document}

begin{equation}

f(x)=begin{cases}

1, & text{if $x<0$}.\
0, & text{otherwise}.
end{cases}
end{equation}
end{document}
That can be achieve in plain LaTeX without any specific package.
documentclass{article}
begin{document}
This is your only binary choices
begin{math}
left{
begin{array}{l}
0\
1
end{array}
right.
end{math}
end{document}
This code produces something which looks what you seems to need.
The same example as in the @Tombart can be obtained with similar code.
documentclass{article}
begin{document}
begin{math}
f(x)=left{
begin{array}{ll}
1, & mbox{if $x<0$}.\
0, & mbox{otherwise}.
end{array}
right.
end{math}
end{document}
This code produces very similar results.