Description-like environment with fixed labels width

You can use the enumitem package to customize the description environment, e.g.

\documentclass{article}
\usepackage{enumitem}
\begin{document}
\begin{description}[leftmargin=8em,style=nextline]
  \item[Something] Text. More text.More text.More text. More text. More text. More text. More text. More text. More text. More text. More text. More text. 
  \item[Ought else] More text.
\end{description}
\end{document}

You can set this for all description environments (probably not desirable) with

\setlist[description]{leftmargin=8em,style=nextline}

or you can define your own list environment e.g.

\newlist{NewDesc}{description}{2}
\setlist[NewDesc]{leftmargin=8em,style=nextline}

and use as

\begin{NewDesc}
  \item[Something] Text. More text.More text.More text. More text. More text. More text. More text. More text. More text. More text. More text. More text. 
  \item[Ought else] More text.
\end{NewDesc}

The KOMA-Script classes and the scrextend package (part of KOMA-Script) provide the labeling list environment. It takes the length of the longest label as mandatory argument.

\documentclass{article}

\usepackage{scrextend}
\addtokomafont{labelinglabel}{\sffamily\bfseries}

\begin{document}

\begin{labeling}{Longer label\quad}
\item[Label] Some text.
\item[Longer label] Some text.
\end{labeling}

\end{document}

Another way, far less elegant to what enumitem is to use a tabular-related structure.

\documentclass{article}
\usepackage{array}% http://ctan.org/pkg/array
\usepackage{tabularx}% http://ctan.org/pkg/tabularx
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\begin{document}
\newcolumntype{L}{@{}>{\bfseries}p{8em}<{:}}% Item label
\newcolumntype{I}{X@{}}% Item contents
\noindent\begin{tabularx}{\textwidth}{LI}
  Label & \lipsum[1] \\
  Longer label & \lipsum[2]
\end{tabularx}
\end{document}

In the above example, tabularx was used to have a table with flexible columns to the maximum text block width (\textwidth). Additionally, the array package provides a means to insert code before/after column entries. This way it is possible to format the first column entries #1 as \bfseries#1: (automatically prepending a bold format and appending a colon :). Finally, the outer column spacing was removed using a @{} column specifier.

The drawbacks are:

• The tabular does not flow across page breaks; and
• It's interface is not as easy as list with special sequences required to distinguish item labels and contents (&) and "new items" (\\).

lipsum merely provided some dummy text.