Secondary professions for a Ranger

Ranger/Warrior
Warrior IAS skills, like Flurry and Flail, and Warrior attack skills that use energy benefit from the Ranger's primary attribute, Expertise.

Ranger/Elementalist
Spells like Conjure Flame are enchantments, and ehe bonus damage is not removed upon activating Barrage.

Ranger/Mesmer
Mesmer skills like Drain Enchantment and Hex Breaker can help fill in gaps in the ranger line for PvP and trapping teams using Mantra of Resolve can avoid interruption.

Ranger/Monk
This profession combination provides little benefit for a Ranger primary.

Ranger/Necromancer
The 'Touch Ranger' takes advantage of expertise's effect on touch-range skills. See Build:R/N Touch Ranger.

Ranger/Assassin
An archery-focused Ranger can employ Shadow Arts skills, particularly reactive Shadow Step skills such as Recall, to keep out of melee range of enemy Warriors, Assassins and Dervishes.

Signet of Malice grants condition removal, protecting against dangerous conditions like blind or cripple.

Ranger/Ritualist
Weapon Spells can work well with a bow, Splinter Weapon in particular combines well with Barrage for heavy AoE damage.

Ranger/Paragon

 * Using Rampage as One with several adrenaline Spear attacks in combination with Apply Poison is an effective way to spread poison and other conditions.

Ranger/Dervish
Rangers with a scythe can be very effective, since Expertise lowers all attack skills up to 48%. A bow ranger can take advantage of enchantments like Ebon Dust Aura, Harrier's Grasp or Attacker's Insight.