High-spin metal complexes have singly occupied orbitals and may not have any empty orbitals into which ligands could donate electron density. In general, there are few or no π-acidic ligands in the complex.  These singly occupied orbitals can combine with the singly occupied orbitals of radical ligands (e.g., oxygen), or addition of a strong field ligand can cause  electron-pairing, thus creating a vacant orbital that it can donate into.
Examples: 
*CrCl(THF) (15 e)
*[Mn(HO)] (17 e)
*[Cu(HO)] (21 e, see comments below)
Complexes containing strongly π-donating ligands often violate the 18-electron rule.  These ligands include fluoride (F), oxide (O), nitride (N), alkoxides (RO), and imides (RN). Examples:
*[CrO] (16 e)
*Mo(=NR)Cl (12 e)
In the latter case, there is substantial donation of the nitrogen lone pairs to the Mo (so the compound could also be described as a 16 e compound).  This can be seen from the short Mo–N bond length, and from the angle Mo–N–C(R), which is nearly 180°.
Counter-examples: 
*trans-WO(MePCHCHPMe) (18 e)
*Cp*ReO (18 e)
In these cases, the M=O bonds are "pure" double bonds (i.e., no donation of the lone pairs of the oxygen to the metal), as reflected in the relatively long bond distances.