Drawing boxes around words

You can use TikZ with the overlay option and a correct anchor. That would look like this:

\documentclass{article}
\usepackage{tikz}
\newcommand\mybox[2][]{\tikz[overlay]\node[fill=blue!20,inner sep=2pt, anchor=text, rectangle, rounded corners=1mm,#1] {#2};\phantom{#2}}
\begin{document}
   \noindent
   this is some text \mybox[fill=blue!20]{box} text\\
   this is some text box text
\end{document}

You can specify extra options (like I have done here for the color). By using the overlay option and the text anchor we ensure correct placement and no influence on spacing. The phantom is added to get the normal spacing for the content of the box. This is the result of the example code:

Edit: To show that the vertical spacing is not affected either, consider the following:

\documentclass{article}
\usepackage{tikz}
\newcommand\mybox[2][]{\tikz[overlay]\node[fill=blue!20,inner sep=2pt, anchor=text, rectangle, rounded corners=1mm,#1] {#2};\phantom{#2}}
\begin{document}
  \begin{minipage}{0.4\textwidth}
    \noindent
    this is some text \mybox[fill=blue!20]{box} text\\
    this is some text box text
  \end{minipage}
  \begin{minipage}{0.4\textwidth}
    \noindent
    this is some text box text\\
    this is some text box text
  \end{minipage}    
\end{document}

Which results in:

My solution needs no special macros. No TikZ, no PSTricks. Only \pdfliteral primitive is used. And you can define \def\pdfliteral#1{\special{pdf:literal #1}} if \pdfliteral isn't available (for example in XeTeX).

\def\mybox#1{\leavevmode \setbox0=\hbox{#1}%
   \dimen0=\wd0 \edef\posxA{\expandafter\ignorept\the\dimen0 \space}%
   \hbox{\kern3pt\pdfliteral{q .8 .8 1 rg .8 .8 1 RG .9963 0 0 .9963 0 0 cm 1 j 1 J 6 w
                             0 0 m 0 5 l \posxA 5 l \posxA 0 l 0 0 l B Q}%
         \box0 \kern3pt}%
}
{\lccode`\?=`\p \lccode`\!=`\t  \lowercase{\gdef\ignorept#1?!{#1}}}

This is a \mybox{test} of my \mybox{box}.

\bye

This kind of boxes can be done with extrude option from tcolorbox. A little example adapted from tcolorbox documentation:

\documentclass{article}
\usepackage[most]{tcolorbox}

\newtcbox{\mybox}[1][]{enhanced, colframe=blue, colback=blue!15, 
       frame style={opacity=0.25}, interior style={opacity=0.25}, 
       nobeforeafter, tcbox raise base, shrink tight, extrude by=1mm, #1}

\begin{document}
Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
Ut \mybox[extrude bottom by=2cm]{purus} elit, vestibulum ut, \mybox{placerat} ac, 
\mybox[extrude top by=2mm]{adipiscing} vitae, \mybox[extrude left by=5mm, extrude 
bottom by=2mm]{felis}. Curabitur dictum gravida mauris. \mybox{Nam} arcu libero,    
nonummy eget, consectetuer id, vulputate a, magna.
\end{document}