The distinctive feature of the Grignard reagents is their formation from the organic halide and magnesium metal.  Most other group II organic compounds are generated by salt metathesis, which limits their accessibility.  The formation of the Grignard reagents has received intense scrutiny. It proceeds by a SET process.  For less reactive organic halides, activated forms of magnesium have been produced in the form of Rieke magnesium. Examples of Grignard reagents are phenylmagnesium bromide and ethylmagnesium bromide. These simplified formulas are deceptive: Grignard reagents generally exist as dietherates, RMgX(ether)2.  As such they obey the octet rule.
Grignard reagents participate in the Schlenk equilibrium.  Exploiting this reaction is a way to generate dimethylmagnesium.  Beyond Grignard reagents, another organomagnesium compound is magnesium anthracene. This orange solid is used as a source of highly active magnesium.  Butadiene-magnesium serves as a source for the butadiene dianion.  Ate complexes of magnesium are also well known, e.g LiMgBu.