The Combat Knife is a rather common melee weapon in Fallout 3. It is affected by the Melee Damage Bonus from Strength. It is one of the better low-level melee weapons, and can get around 3-4 strikes in VATS even with a reasonable number of AP and a fast swing speed outside of VATS. However it has extremely short reach, about that of an Unarmed weapon.
The combat knife itself has a single edge with a clipped point, with a serrated edge on the back. It is of the standard Bowie shape, and is really more of a utility knife than an actual weapon.
Stabhappy - Stabhappy shares all of its stats with Occam's Razor, but has two principle differences. Firstly, the raider that holds it respawns, allowing infinite copies to be obtained. Stabhappy also deals an additional 50% damage to limbs.
The name of the Combat Knife in the description of Fallout, Fallout 2 and Fallout Tactics could be a reference to Sylvester Stallone's role as Rambo in the First Blood series. Rambo was known for carrying a large combat knife. (The name of the company, "SharpWit"'s, double meaning also seems to point to this.)
The Combat Knife of Fallout 3 seems similar in appearance to the KA-BAR combat knife that is currently used by most branches of the United States Military but there are several differences. KA-BAR's are not serrated, they have a differently shaped fuller (aka: blood groove) and they have a black finish (although it could have just worn off after 200 years of abuse).