Space of Imperative Programs
The Lattice of Flow Diagrams, Dana Scott, Programming Research Group report 03, Oxford University, 1969.

Flow diagram = Control flow graph. Another viewpoint is of  imperative programs as generators of possibly infinite transition systems. The transition systems will form a coalgebra.

Introduction to Coalgebra. Towards Mathematics of States and Observation, Bart Jacobs. 

Artem's comment and Neel's answer provide further possibilities. There are many answers to this question depending on what you mean by imperative program and what properties are of interest. Do you care only about syntactic objects or some underlying semantic notion? Even if you only care about, say, control flow graphs, there is a difference between arbitrary flow graphs and those generated by standard structured programming constructs (the latter are reducible). These properties in turn affect the mathematical structure of the space involved.