align[Right] = {1, 0};
align[Center] = {0, 0};  (* default *)
align[Left] = {-1, 0};

lText = Text["Left Aligned", {0.5, 0.6}, align[Left]];
cText = Text["Centered", {0.5, 0.5}, align[Center]];
rText = Text["Right Aligned", {0.5, 0.4}, align[Right]];

txt = Graphics[{lText, cText, rText}];
plot = Plot[Sin[π t], {t, 0, 1}];

Show[plot, txt]