Captions apart from Floats
If you need your floats on own pages only with a caption like "Figure #" and additionally a list of corresponding captions, try the following (needs endfloat.sty):

write in preamble:

\usepackage{endfloat}
\let\Caption\caption
\renewcommand\caption[1]{%
    \Caption[#1]{}%
}

\caption[short for toc]{long for caption}

Left aligned Captions
Write in preamble:

\makeatletter
\long\def\@makecaption#1#2{%
  \vskip\abovecaptionskip
  \sbox\@tempboxa{#1: #2}%
  \ifdim \wd\@tempboxa >\hsize
    #1: #2\par
  \else
    \global \@minipagefalse
    \hb@xt@\hsize{\box\@tempboxa\hfil}%
  \fi
  \vskip\belowcaptionskip}
\makeatother

Center last Line
It's no problem to center the caption, but this doesnn't happens for the last line, which is still left aligned. Write in preamble:

\usepackage{ccaption}
\captionstyle{\centerlastline}

Linespacing in Captions
Documentwide spacing is not important for captions. If you want to change the linespacing in captions too, write in preamble:

\usepackage{setspace}
\let\myCaption\caption
\renewcommand\caption[1]{%
  \doublespacing
  \myCaption{#1}
}

Caption over/under floats
Sometimes the captions for tables are above and for figures under the float. This gives a bad layout, because the values for \abovecapatinskip and \belowcaptionskip are not the same. In this case you can change the values for the tables with a caption over the float. Write in latex preamble:

\makeatletter
\newcommand{\@ldtable}{}
\let\@ldtable\table
\renewcommand{\table}{%
                 \setlength{\@tempdima}{\abovecaptionskip}%
                 \setlength{\abovecaptionskip}{\belowcaptionskip}%
                 \setlength{\belowcaptionskip}{\@tempdima}%
                 \@ldtable}
\makeatother 

Vertical space over/under captions

\setlength{\abovecaptionskip}{0.5cm}   % 0.5cm as an example
\setlength{\belowcaptionskip}{0.5cm}   % 0.5cm as an example

captionlayout

can be changed easy with the package caption, available at CTAN for download.
Example:

\usepackage[it]{caption}    % captionfontattribut italic
\renewcommand\captionmargin{1cm}  % new left and rightmargin

\usepackage{ccaption} % that is double c
\captionnamefont{\bfseries}
\captiontitlefont{\bfseries} 

\newcommand\myCaption[1]{\small\refstepcounter{figure}%
   \centering\figurename\ \thefigure :\ #1}

\myCaption{...the captiontext ...}