\documentclass{article}
\usepackage{pgfplots}

\pgfplotsset{width=7cm,compat=1.5.1}

\begin{document}

\begin{tikzpicture} 
\begin{axis}[scatter/classes={
    a={mark=square*,blue},%
    b={mark=triangle*,red},%
    c={mark=o,draw=black}}]
    %  \addplot[] is better than \addplot+[] here:
    %  it avoids scalings of the cycle list
    \addplot[scatter,only marks,
        scatter src=explicit symbolic]
        coordinates {
(0.1,0.15) [a] 
(0.45,0.27) [c] 
(0.02,0.17) [a] 
(0.06,0.1) [a] 
(0.9,0.5) [b] 
(0.5,0.3) [c] 
(0.85,0.52) [b] 
(0.12,0.05) [a] 
(0.73,0.45) [b] 
(0.53,0.25) [c] 
(0.76,0.5) [b] 
(0.55,0.32) [c]
        };
\end{axis}
\end{tikzpicture}

\end{document}