The linegoal package provides a macro \linegoal to be used with \setlength:
\setlength<
will set <some dimen> to the horizontal length of the remainder of the line.

This is achieved using the \pdfsavepos primitive of pdftex, through the package.

Example:

Some text: \begin{tabularx}\linegoal{|l|X|} \hline
             one & two \\
	     three & four \\\hline
	   \end{tabularx}

will position the table after the initial text, and make the table fill the rest of the line.

The author is Florent Chervet. The package is Copyright © 2010 Florent Chervet.