Skills that give +ar
armor - no player classes
stone skin - Ranger, Wizard (Wizard through 'conjure elemental' ooze)
shield - wizard, music
elemental shield - shaman
ice invoction - druid
ooze invoction - druid
force field - psionic
shadow armor - witch
lightning shroud - archon
telekinetic shield - psionic
guise of nature - ranger (at level 240/241 gives +4 ar. At lvl 55 +1 ar)
monument - archon religion spell
aegis - paladin
combat blink - monk
The following spells will only allow you to have one cast on you at one time
armor, guise of nature
ice invocation, shield, force field
ooze invocation, stone skin