Secondary professions for a Warrior

Warrior/Ranger

 * Pets can be used in conjunction with "I Will Avenge You!" for decent unstrippable IAS and health regeneration.
 * Antidote Signet is unlinked, free condition removal.

Warrior/Monk

 * Mending Touch can be useful for removing conditions
 * The Monk secondary profession opens up several useful Resurrection skills, for use in PvE.

Warrior/Necromancer

 * The Warrior/Necromancer combination offers very little in the way of usefulness.

Warrior/Mesmer

 * Wise use of Physical Resistance, Elemental Resistance or elemental mantras can add additional survivability to solo or tanking builds.

Warrior/Elementalist

 * Shock can be used as a snare and interrupt, however exhaustion will certainly be felt with a Warrior's low energy pool.
 * Grasping Earth can be used as an AoE snare to keep groups of enemies from kiting as efficiently.
 * The conjure skills, such as Conjure Lightning can provide additional damage when wielding an elemental weapon for a very small investment of attribute points.

Warrior/Assassin

 * Signet of Malice can provide free condition removal.
 * Dash is an excellent speed boost that also serves as a quickly recharging cancel stance for Frenzy.
 * Death's Charge is useful for closing distance in battle, but take care not to outrun your party's support.

Warrior/Ritualist

 * Various skills can be used in Warrior/Ritualist farming builds, such as Vengeful Was Khanhei and Vengeful Weapon.
 * Death Pact Signet can be useful for resurrecting your monks in high-end PvP such as GvG.

Warrior/Paragon

 * This combination can be used for the Flail Spear build, using the IAS skill Flail and high attributes in Strength and Spear Mastery to deal damage.
 * Enduring Harmony can be quite useful for extending warrior shouts such as "For Great Justice!"
 * In HA, warriors sometimes equip Song of Concentration to help their Ghostly Hero cap the altar.

Warrior/Dervish

 * Many Scythe Mastery skills are general-purpose Melee Attacks (i.e. Rending Sweep) rather than weapon-specific skills, which allows them to be used with axes, hammers, and swords.
 * Warrior's Endurance can be used effectively with a scythe and energy-based attacks, such as Power Attack or Eremite's Attack, along with an IAS to allow for excellent DPS. In PvE, Aura of Holy Might may be used to further increase damage.