General secondary professions

Warrior

 * "Save Yourselves!" can be used in PvE to briefly provide allies with a brief but powerful armor buff.

Ranger

 * Antidote Signet is a free, fast recharging condition removal skill. It can only remove Blindness, Disease and Poison, however it can remove all three in one activation.
 * Serpent's Quickness is useful for reducing skill recharge times.
 * Troll Unguent is cheap, provides considerable health regeneration and cannot be removed.

Monk

 * Deny Hexes, Holy Veil, Remove Hex and Smite Hex are all 5 energy hex removal skills.
 * Martyr can be used to transfer the conditions of all party members.
 * Mending Touch is cheap, recharges quickly and can remove two conditions in a single casting, as well as providing some healing.
 * Purge Conditions is fast and removes all conditions in a single casting, however it has a much longer recharge time than Mending Touch.
 * Purge Signet can be used to remove stacked conditions and hexes, however it can quickly drain your energy pool.
 * Having a reusable resurrection skill is often helpful in PvE. Rebirth is particularly useful for recovering from a near-wipe.

Necromancer

 * Gaze of Contempt and Rend Enchantments are useful mass enchantment removal skill.
 * Signet of Corruption can be used in PvE as an energy management skill.

Mesmer

 * The Inspiration Magic attribute includes numerous energy management skills.
 * Drain Enchantment, Inspired Enchantment, Lyssa's Balance, Revealed Enchantment and Shatter Storm are cheap enchantment removal skills.
 * Echo can be effectively utilized by any profession.
 * Elemental Resistance, Physical Resistance, Mantra of Earth, Mantra of Flame, Mantra of Frost, Mantra of Lightning are useful in areas where most or all phoes use a particular damage type.
 * Epidemic can cheaply spread conditions among foes.
 * Hex Eater Signet, Inspired Hex and Revealed Hex are cheap hex removal skills, and Hex Breaker is a cheap hex prevention skill.
 * Mantra of Inscriptions can be used to improve the recharge time of Signets.
 * Power Drain, Power Return, Leech Signet and Signet of Disruption are cheap interruption skills.

Assassin

 * Shadow step skills can be useful for any profession.
 * Assassin's Promise can be an effective energy management and recharge reduction skill.
 * Crippling Dagger is a cheap, ranged snare skill.
 * Disrupting Dagger is a cheap, ranged interruption skill.
 * Signet of Malice is an effective condition removal skill.

Ritualist

 * Having a reusable resurrection skill is often helpful in PvE.

Paragon

 * Cautery Signet can be used to remove conditions from the entire party.
 * Remedy Signet is a free, fast recharging condition removal skill.

Warrior

 * Whirlwind Attack is a multi-target melee attack, and it is an adrenal skill, so it is effectively "free" when used alongside Energy-using Assassin and Dervish skills.
 * Wild Blow can be used to remove stances.

Necromancer

 * Plague Touch offers a cheap, fast way to get conditions off.

Mesmer

 * Illusionary Weaponry can provide considerable melee DPS, although it is vulnerable to enchantment removal.

Elementalist

 * Conjure Flame, Conjure Frost and Conjure Lightning are useful damage buffs when combined with the corresponding elemental damage weapon mod.

Dervish

 * Ebon Dust Aura can be used with an earth damage weapon or Dust Cloak in order to blind foes.
 * Rending Touch is a cheap touch-range enchantment removal skill, which has no drawback if you are not enchanted.

Ranger
See Paragon / Ranger in secondary professions for a Paragon.

Elementalist

 * Conjure Flame, Conjure Frost and Conjure Lightning are useful damage buffs.

Paragon
See Ranger / Paragon in secondary professions for a Ranger.

Monk

 * Against conditions other than Burning, secondary Monks can use Extinguish as effectively as primary Monks.

Necromancer

 * Enfeebling Blood, Reckless Haste and Shadow of Fear are effective multi-target anti-melee skills.
 * Necrosis can be used in PvE to spam damage on a single target.

Mesmer

 * Cry of Pain can be used in PvE to interrupt a foe and inflict area damage.
 * Ether Nightmare can be used in PvE to inflict degeneration on multiple targets.
 * Mantra of Concentration and Mantra of Resolve are useful for preventing interrupts.
 * Mirror of Disenchantment can be used as effectively be secondary Mesmers as by primary Mesmers.

Elementalist

 * Blurred Vision is an effective multi-target anti-melee hex.
 * Intensity can be used in PvE to provide a brief spell damage buff.
 * Many Glyphs are both unlinked and equally useful for both Elementalist and non-Elementalist spells.

Assassin

 * The PvE-only skill Shadow Sanctuary is cheap and functions as both a self-heal and a defensive skill, and the drawback of blindness generally has little impact on casters.