Prepending a constant value to sublists of a list

Rewriting other's using operator forms

I like @eldo's solution, but it can be shorter!

Map[Prepend[-1], list, {2}]

Same for @aardvark2012's

Map[Insert[-1, 1], list, {2}]

Solutions of my own

list[[All, All, 0]] = Prepend[-1]@*List;

Or better

list[[All, All, 0]] = {-1, ##} &;

Also as an operator

ReplacePart[{_, _, 0} -> ({-1, ##} &)]

Best other

Given that the OP accepted mine (hastily to my taste and despite the good quality of the others), I want to highlight this answer given by Mr.Wizard in a comment to another answer. Easy to understand, and short:

Map@Prepend[-1] /@ list

☺ = {-1, ##} & @@@ # & /@ # &; 

☺ @ list

(* thanks: Mr.Wizard *)

If we're golfing ;-)

Prepend[-1]/@#&/@list