  We consider the effects of heavy Majorana neutrinos N with sub-TeV masses. We
argue that the mere presence of these particles would be a signal of physics
beyond the minimal seesaw mechanism and their interactions are, therefore, best
described using an effective Lagrangian. We then consider the complete set of
leading effective operators (up to dimension 6) involving the N and Standard
Model fields and show that these interactions can be relatively easy to track
at high-energy colliders. For example, we find that an exchange of a TeV-scale
heavy vector field can yield thousands of characteristic same-sign lepton
number violating l^+ l^+ j j events (j=light jet) at the LHC if m_N < 600 GeV,
which can also have a distinctive forward-backward asymmetry signal; even the
Tevatron has good prospects for this signature if m_N < 300 GeV.
