Test::More, Test::Distribution (staring to use this), Pod::Coverage
Parse::RecDescent
Imager, GD
many modules in the Math::* section since I'm developing Math::* modules myself pretty actively. Examples: Math::Complex, Math::Approx, Math::Spline, Math::Big*, Math::MatrixReal, etc.
perltidy (or Perl::Tidy), well, at least indirectly.