What skills in Diablo 2 can be cast on friendly players, summoned creatures and hirelings?
Holy Bolt (Paladin) and Enchant (Sorceress) are the only spells that can be directly cast on allies and allied minions (i.e. actually targeting the ally with the spell).
Three Barbarian warcries can be used which affect all allies who are present when the skill is cast: Shout, Battle Orders, and Battle Command.
I think those are the only which actually qualify as being "cast on friendly players", although there are other skills which act as buffs you can give to friendly players as long as they are close to you (passive auras, although in some cases the aura needs to be kept on an active skill slot). They lose the buffs as they go out of range, and automatically regain the buff when they are in range again.
There are quite a few aura skills for the Paladin which, when active, affect all nearby allies: Resist Fire, Resist Cold, Resist Lightning, Cleansing, Vigor, Meditation, and Salvation all of the Defensive Auras tree, as well as Might, Thorns, Blessed Aim, Concentration, and Fanaticism from the Offensive Auras tree.
The Druid has a couple spells which summon creatures which radiate auras: Oak Sage, Heart of Wolverine, and Spirit of Barbs
Some of these can also be granted by items or act 2 mercs.