Fin-like protrusions on bottom of vertical stabilizer.

[SX737]

Could anybody solve the mystery on what exactly are these fin-like protrusions on the bottom of vertical stabilizer??!

[411A]

..on exactly what you refer to as 'fin like protrusions'.

On old models of the Boeing 707 (with straight-pipe, non-fan engines) these are called votex generators, and were designed to provide boundary layer excitation, to improve the elevator response/effectiveness.

In essence, they correct for design abnormalities experienced in certification flight testing.
Oftentimes these are also noticed on the top surface of the wing as well, usually just forward of the inboard and/or outboard ailerons.

[Milt]

The fins you describe are usually add ons to correct for inadequate directional stability of the prototype.

Seperate add on fins are called strakes which are usually positioned under the tail.

[PaperTiger]

Taking a stab based on your userid, those are vortex generators at the back of a 737: http://www.b737.org.uk/fuselage.htm

[low n' slow]

Strakes = longish fins producing a stable vortex in certain AoA and otherwise nothing but slight drag. The MD80 has a pair on the nose that provide the fin with two stable vortices on either side at high AoA and thereby giving it extra horizontal stability with a high nose.

Vortex Generators = knifelike protrusions allways at an angle to the normal airflow. Induces a high energy boundrylayer over surfaces where airflow adherance is valued higher than low friction. Can be a disigners quick remedy for loss of aileron effectiveness at slow speeds.

Finlike protrusions at the botom of the vertical stabilizer = ?
I believe you mean the little fins found on for example the beech 1900. I dont quite know, but they seem to be in line with the enginge naceles. It's quite possible that these produce some turbulence around the stabilizer in high AoA and there is a need to take control of this disorderly airflow to get predictable characteristics. The same problem is found on most seaplanes using two floats for flotation. At high AoA they infringe the airflow around the stabilizer and whole empennage. This gives bad flying characteristics when flying with a high attitude. The piper cub uses an add-on fin under the body to remedy this.

regards/lns