\usetikzlibrary{arrows}

\begin{tikzpicture}[font=\scriptsize]
\node (a) at (0,0) [circle, draw] {a};
\node (b) at (1,1) [circle, draw] {b};
\draw (a) to[bend left] (b)
		   (a) to[bend right] (b);
\draw [dashed] (a) |- (b);
\draw [red,->>] (a) -| (b);
\draw [xshift=2pt] (a) -- (b);
\draw [xshift=-2pt] (a) -- (b);

    \node[rectangle,draw] (A) at (.5,.5) {A};
    \node[rectangle,draw] (B) at (3,1.5) {B};
    \node[rectangle,draw] (C) at (.5,-2) {C};
    \draw[-left to] (A.north east) -- (B.west);
    \draw[-left to] (B) -- (A);
	\draw[->] (A.280) -- (C.80);
	\draw[<-] (A.260) -- (C.100);
\end{tikzpicture}

various arrow tricks between nodes in tikz

LaTeX