Since it is basically how they appear in-game, I'm going to use a different format from the one used before.
Sticky (boots + BT) - -90% knockback
Springloaded (boots + AT) - +50% knockback, move two spaces per step
Sawed-Off (any shotgun + PT) - damage boost (+2d), larger damage spread, faster drop-off rate[1]
Shield Sword (combat knife + AB) - adds 2 armor, -25% knockback, damage stays 2d5
Nuclear (any ranged weapon + NT) - changes alt. reload to "overload", causing a nuclear explosion w/ 10-second countdown sequence.[2]
Piercing (bullet weapon + PTT) - changes damage type to plasma
Flak Shotgun (combat shotgun + PPB) - alternate fire type, burst: fires two shots simultaneously, at 1.5x firing time
Vertigo Cannon (plasma rifle + PAT) - enemies that are hit by shots are "disoriented" for 0.3sec per shot, stacks[3]
Expandable (armor + BBB) - can store extra ammo while equipped[4]
Incendiary (rapid fire/rocket launcher + TTF) - reduces attack damage to 1d1 per shot; adds "fire" damage, dealing four pure damage per second[5]
--rapid fire: hurts for one second per shot, stacks up to 8 seconds
--rocket launcher: lasts five seconds, affects everything in splash
C4 Launcher (rocket launcher + PAA) - precision attack (lands on target tile); shot remains "frozen" until detonated; alternate fire type, detonate: all frozen shots are activated and deal damage
[1]: This will effectively turn a combat shotgun into a normal, a normal into a double, and a double into a VERY close-range shotgun. It doesn't affect the maximum range of the weapon, however, so although you'll be hurting less at farther distances, you'll still hit and probably aggravate stuff the same way.
[2]: With Nano, this turns any weapon into an equivalent "exotic" nuclear weapon. Sometimes you need a nuke: this is just another way to get one.
[3]: Disorientation just means if it tries to move or attack, it will do so in a random direction. Alternatively, to prevent being WAY too awesome (hard to say without testing) attacking could just incur a -50% to-hit penalty (or the number equivalent).
[4]: Not too sure how this would be implemented, whether a Backpack-like effect would work or if if the armor itself just has "storage space" for, say, four or five slots that can only be used with ammo.
[5]: For obvious reasons, fire damage doesn't affect Arch-viles.