Secondary professions for a Mesmer

Mesmer Warrior
Illusionary Weaponry with some combination of the IAS Warrior skills provides ample pressure damage. Mesmers can also take advantage of superior energy regeneration to use the more energy-intensive skills within the Tactics line to mitigate damage.

Mesmer Ranger
This profession combination provides little benefit for a Mesmer primary.

Mesmer Elementalist
The Mesmer/Elementalist can take advantage of the Mesmer's Fast Casting attribute with the Elementalist's high damage spells to create large amounts of damage until their energy is depleted.

Fast Casting can be used to take advantage of some of the timing-based effects offered by the Elementalist class, such as knockdowns with Gale or ensure Aftershock conditional damage.

Mesmer Monk
Since the slow-casting resurrection spells of a Monk are easily interrupted, some Mesmers go with a monk secondary in GvG bringing a resurrection spell and also putting attribute points into Fast Casting to allow for faster resurrecting. This is often referred to as a "Rezmer" and is one of the more specialized uses of a Mesmer.

Because Mesmers have many skills that affect Signets, they can dish out fast damage and support using Monk Smiting Prayers Signets.

Mesmer Necromancer
Me/N is a profession combination usually associated with specialization in status effects like hexes. This mix has access to a wide range of hexes from both professions.

Soul Barbs compounds the damage of the numerous Mesmer hexes. Fragility along with several Necromancer spells that cause conditions is a powerful spike.

Mesmer Assassin
Daggers have a very fast attack speed and are a good way to do a lot of Illusionary Weaponry damage. Assassins also have a number of hexes that can expand a mesmer's effect, such as Shadow Shroud to assist spikes. Aura of Displacement and Blackout could be used to shutdown an enemy monk from afar while being able to safely teleport back to your group.

Mesmer Ritualist
Fast Casting affects both Item Spells and Weapon Spells, but not Binding Rituals. Fast-casting some of the powerful but slow Channeling spells, such as Spirit Rift, can also be effective.

Mesmer Paragon
This profession combination provides little benefit for a Mesmer primary.

Mesmer Dervish
The Mesmer/Dervish typically focuses on using Illusionary Weaponry in conjunction with a scythe.