  Using 281 pb^-1 of data collected with the CLEO-c detector, we report on
first observations and new measurements of Cabibbo-suppressed decays of D
mesons to 2, 3, 4, and 5 pions. Branching fractions of previously unobserved
modes are measured to be: B(D^0\to
pi^+pi^-pi^0pi^0)=(9.9\pm0.6\pm0.7\pm0.2\pm0.1)x10^-3,
B(D^0\to\pi^+\pi^+\pi^-\pi^-\pi^0)=(4.1\pm0.5\pm0.2\pm0.1\pm0.0)x10^-3,
B(D^+\to\pi^+\pi^0\pi^0)=(4.8\pm0.3\pm0.3\pm0.2)x10^-3,
B(D^+\to\pi^+\pi^+\pi^-\pi^0)=(11.6\pm0.4\pm0.6\pm0.4)x10^-3,
B(D^0\to\eta\pi^0)=(0.62\pm0.14\pm0.05\pm0.01\pm0.01)x10^-3, and
B(D^0\to\omega\pi^+\pi^-)=(1.7\pm0.5\pm0.2\pm0.0\pm0.0)x10^-3. The
uncertainties are from statistics, experimental systematics, normalization and
CP correlations (for D^0 modes only). Improvements in other multi-pion decay
modes are also presented. The D-->pi pi rates allow us to extract the ratio of
isospin amplitudes A(Delta I=3/2)/A(\Delta
I=1/2)=0.420\pm0.014(stat)\pm0.016(syst) and the strong phase shift of
delta_I=(86.4+-2.8+-3.3) degrees, which is quite large and now more precisely
determined.
