General secondary professions

Warrior

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

Ranger

 * Antidote Signet is a free, fast recharging condition removal skill. It can remove Blindness, Disease, Poison and one additional condition.
 * Serpent's Quickness is useful for reducing skill recharge times.

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 allies in radar range.
 * 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 skills.

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 and Mantra of Lightning are useful in areas where most or all foes 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.

Elementalist

 * Glyph of Lesser Energy is helpful when casting spells with high energy cost. Even at 0 Energy Storage it saves you 10 Energy.

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.

Ritualist

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

Paragon

 * Remedy Signet is a free, fast recharging condition removal skill.
 * Many shouts and chants are useful, even with minimal points invested into the linked attribute, simply for their party-wide benefits.

Dervish

 * Mystic Regeneration is helpful in enchantment-based builds with a decent investment in Earth Prayers.

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.

Ranger

 * Apply Poison is a an easy way to afflict poison on your targets. No Warrior or Dervish skills directly inflict poison, and most Assassin skills that deal this condition hold prerequisites.

Monk

 * Mending Touch offers a fast, inexpensive, quick-recharging method of removing conditions from yourself.

Necromancer

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

Mesmer

 * Hex Breaker is a stance that allows you to block an incoming Hex.

Elementalist

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

Assassin

 * Shadow stepping skills can be used to quickly close to melee range.

Ritualist

 * Several Weapon Spells can be helpful in fighting large mobs, or as a cheap heal.

Paragon

 * The shout Fall Back is helpful after a battle when running to a new target.
 * Anthem of Guidance, Anthem of Envy, and Anthem of Fury are all good melee character skills. Guidance guarantees a hit, Envy is extra damage at the beginning, and Fury is free adrenaline to get a combo started.

Dervish

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

Warrior

 * The Warrior secondary has IAS skills such as Flail and Tiger Stance that are good alternatives for ranged classes.

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

Elementalist

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

Assassin

 * Skills used to shadow step to an ally (such as Return) can be used to kite opponents or keep them at your preferred range.

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

Dervish

 * Ebon Dust Aura can be used with an earth damage weapon, Staggering Force or Dust Cloak in order to blind foes.

Warrior

 * Blocking stances such as Shield Stance can be used to defend against melee attackers in both PvE and PvP when it is expected the player will be targeted.

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 and Arcane Echo can be used as effectively by 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. Additionally, Glyph of Lesser Energy, altohugh linked to the Elementalist's primary attribute, is effective even without any ranks of Energy Storage.

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.
 * Feigned Neutrality can be used as a self healing skill, and grants an armor bonus.
 * Dark Escape provides damage reduction.

Ritualist

 * Mighty Was Vorizun is an easy way to get an extra 30 energy.

Paragon

 * Skills such as Aria of Zeal provide ample party-wide energy restoration for a net cost of nothing.
 * Lyric of Zeal works great with Signet users.