I noticed the original melee stat dropped from the original game, and added it back in for X@COM, so pretty much *any* item can be used for a melee attack.
The default data I'm using right now (not necessarily balanced properly) has most items with 100% base accuracy (note that by X-COM rules that doesn't mean you always hit) and take 30 TU (unique melee items and terror unit melee attacks are different, of course). It only works if you are adjacent to the enemy, duh. (Unless you're using a throwing weapon like the throwing knives, but those are really designed using projectile weapon properties.)
The damage done by the default melee attacks depends on the attacker's strength, though again this can be overridden for special items. I also added some standard damage types (bludgeoning/slashing/piercing) that can be used for melee attacks.
Not as deadly as actually shooting a weapon (makes sense), of course, but it allows for lots of fun new mechanics, like dedicated melee weapons (e.g. knives, light sabres, etc.) and bashing down weak terrain (or even sturdy terrain like walls using a sledge hammer!). Melee attacks also cause knockback on the target if your strength is greater than theirs, and if your strength is also great enough to smash through whatever is behind the defender, then you'll smash them right through it. So you can, for example, whack an alien with a baseball bat and send him flying out a third-story window so he hopefully falls to his death from impact damage
In fact, if you had superhuman strength, you could theoretically smash aliens right through walls, and if it collapsed part of the structure the debris would even fall on them, which would be pretty funny... Of course, you have to watch out for powerful aliens doing the same things to you
There are other fun melee mechanics, but they go a bit further from X-COM territory (ex: volatile items that explode on contact, including melee impact).
