Implemented code to compute pathwidth (= Node search number, vertex separation number, interval thickness)
Don't know about "an implementation" but check out

Computing Pathwidth Faster Than 2^n 
Karol Suchan and Yngve Villanger 
Parameterized and Exact Computation, 4th International Workshop, IWPEC 2009,Copenhagen, Denmark, Springer Verlag, Lecture Notes in Computer Science 5917, Pages 324-335.