How to change spaces between items in Table of Contents

There are so many options... without further information, one possible option would be to use the setspace package and one of its commands of environments; for example,

documentclass{report}
usepackage{setspace}

begin{document}

doublespacing
tableofcontents
singlespacing

chapter{Test Chapter One}
section{Test Section One One}
section{Test Section One Two}
chapter{Test Chapter Two}
section{Test Section Two One}
section{Test Section Two Two}

end{document}

This approach will increase evenly the space between all sectional units. The following three options allow you to control separately the spacing for each group of sectional units.

Another option would be to use the tocloft package and redefine the cftXafterpnum family of commands:

documentclass{report}
usepackage{tocloft}

renewcommandcftchapafterpnum{vskip10pt}
renewcommandcftsecafterpnum{vskip15pt}

begin{document}

tableofcontents

chapter{Test Chapter One}
section{Test Section One One}
section{Test Section One Two}
chapter{Test Chapter Two}
section{Test Section Two One}
section{Test Section Two Two}

end{document}

Yet another option, this time using the etoolbox package to patch the sectional units commands to add vertical space to the ToC:

documentclass{report}
usepackage{etoolbox}

makeatletter
pretocmd{chapter}{addtocontents{toc}{protectaddvspace{15p@}}}{}{}
pretocmd{section}{addtocontents{toc}{protectaddvspace{5p@}}}{}{}
makeatother

begin{document}

tableofcontents

chapter{Test Chapter One}
section{Test Section One One}
section{Test Section One Two}
chapter{Test Chapter Two}
section{Test Section Two One}
section{Test Section Two Two}

end{document}

A fourth option not requiring any packages would be to redefine the chapter and section commands as implemented in the used document class to add the vertical space to the ToC; an example with report:

documentclass{report}

makeatletter
renewcommandchapter{addtocontents{toc}{protectaddvspace{5p@}}%
  if@openrightcleardoublepageelseclearpagefi
  thispagestyle{plain}%
  global@topnumz@
  @afterindentfalse
  secdef@chapter@schapter}
renewcommandsection{addtocontents{toc}{protectaddvspace{20p@}}%
  @startsection {section}{1}{z@}%
  {-3.5ex @plus -1ex @minus -.2ex}%
  {2.3ex @plus.2ex}%
  {normalfontLargebfseries}}
makeatother

begin{document}

tableofcontents

chapter{Test Chapter One}
section{Test Section One}
section{Test Section Two}
chapter{Test Chapter One}
section{Test Section One}
section{Test Section Two}

end{document}