Secondary professions for a Ranger

Ranger/Warrior
A Warrior secondary profession combines well with a Ranger primary. Rangers can utilize several Warrior IAS skills (Flurry, Flail), and Warrior attack skills that use energy (which will benefit from the Ranger's primary attribute, Expertise).

Ranger/Elementalist
In PvE, R/E's can take advantage of the Conjure spells to enhance their damage output. This technique is usually employed for use with Barrage. Since Barrage will remove any preparations upon use, relying on Ignite Arrows or Kindle Arrows for extra damage is a waste. However, since spells like Conjure Flame or Conjure Frost are enchantments the bonus damage is not removed upon activating Barrage. Enchantments have the additional advantage of lasting longer than preparations.

Ranger/Mesmer
R/Me is a very common profession combination in PvP. It is used for the Ranger spike teams (using the Mesmer skills Drain Enchantment and Hex Breaker) and trapping teams (using Mantra of Resolve to avoid interruptions while setting traps).

Ranger/Monk
The Ranger/Monk (R/Mo) is a very flexible profession combination. It is often used as a trapper in The Underworld, usually concentrating on Expertise and Wilderness Survival attributes.

The R/Mo can be used as a runner, combining the speed skills of the Ranger and the hex and condition removal of a monk.

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 particularly effective.

Ranger/Dervish
Ranger primaries can use a scythe. Since all attacks are Energy-based, and Expertise lowers them up to 48%. A few skills could be useful for runners, like Hex removers and Vow of Silence. A bow ranger can take advantage of enchantments like Ebon Dust Aura, Harrier's Grasp or Attacker's Insight used with high-energy skills like Broad Head Arrow and Concussion Shot.