Disciple of Dispater, from Book of Vile Darkness (which is 3.0), has an ability called Iron Power, which, among other things, doubles (at 4th level) and triples (at 8th level) the weapon's crit range. It states that it stacks with Improved Critical, but not with the Keen property.
Hrm...actually, with flaws, it's possible to effectively combine Disciple of Dispater and Weapon Master in 20 levels. Say, Fighter-5, Disciple of Dispater-1, Weapon Master-7, and Disciple of Dispater 2-8. At that point, you've got a crit range of 7-20 with your falchion, which is truly disgusting.
Dodge (1st level)
WF: Falchion (Ftr-1 bonus)
Combat Expertise (Human bonus)
Combat Reflexes (Ftr-2 bonus)
Mobility (3rd level)
Spring Attack (Ftr-4 bonus)
Whirlwind Attack (6th level) (Note that 6th level is DoD-1)
WS: Falchion (9th level)
Improved Critical: Falchion (12th level)
15th-level and 18th-level feats are open.
Choose two flaws (ideally at 1st level), and take these as your bonus feats:
Power Attack (Flaw)
Disciple of Darkness (Flaw)
Of course, without modifications, Disciple of Dispater really isn't useful for PCs. That whole "sold soul to archdevil and sacrifice a sentient being" thing. However, there's probably a good BBEG use for this build. My DM used a version of this way back in 3.0 on us, with a Vorpal Falchion, and boy, was that fun.
Brad