Aligning numbers by decimal points in table columns

The newest option is using the S column type of the siunitx package.

\documentclass{article}

\usepackage{siunitx}

\begin{document}
\begin{tabular}{S[table-format=3.2]}% syntax for siunitx v2; for v1 use "tabformat"
555 \\
7.77 \\
99.9
\end{tabular}

\end{document}

Even though it's already been mentioned in the posting, it's worth discussing the dcolumn package in more detail. The package provides a column type called D that performs alignment on the decimal marker. The D column type takes three inputs: the input decimal marker (usually . or ,), the output decimal marker (again, usually . or ,), and the number of digits before and after the decimal marker. If the input and output decimal markers are always the same -- e.g., always . -- it's useful to set up a short-hand as follows:

\newcolumntype{d}[1]{D{.}{.}{#1}}

Here's a full MWE that's based on @lockstep's code -- indeed, with this simple example the outputs of S[table-format=3.2] and D{.}{.}{3.2} are identical :

\documentclass{article}
\usepackage{dcolumn}
\newcolumntype{d}[1]{D{.}{.}{#1}}
\begin{document}
\begin{tabular}{d{3.2}}
555 \\
7.77 \\
99.9
\end{tabular}
\end{document}