Secondary professions for an Assassin

Assassin/Warrior
Due to their base attack speed, Assassin/Warriors get a lot of adrenaline, so it's possible to use adrenaline skills to great affect. Skills such as Rush or "Fear Me!" are good examples: while attacking a target, you can continuously Rush. Fear Me is a great energy drain, but it requires investment in Tactics to yield results.

Assassins do not have any attack speed enhancing stances, and can thus greatly benefit from Warrior skills such as Flurry and Burst of Aggression. Burst of Aggression can be particularly useful, as it has no negative drawback for its speed boost other than the loss of adrenaline, which Assassins do not often use. Example Builds:
 * Build:A/W Critical Mastery

Assassin/Ranger
Barrage damage can be improved with high critical strikes attributes, adding bonus damage to attacks, energy bonus, and effects from skills like Way of Perfection.

Example Builds:
 * Build:A/R Critical Barrager
 * Build:A/R Solo Rajazan's Fervor Farmer

Assassin/Elementalist
Example Builds:
 * Build:A/E Falling Shocker
 * Build:A/E Signet Spiker

Assassin/Mesmer
The Mesmer skill Blackout can be used while all of your attack skills are recharging without penalty, and can also be employed in combination with Temple Strike to further lock down a foe.

The Assassin Mesmer is excellent at running, especially with a buddy; with a 20% enchantment item and full attribute points in Shadow Arts, Shadow Form coupled with Arcane Echo and if with a friend, Arcane Mimicry, can produce a mostly unstoppable force.

Example Builds:
 * Build:A/Me Solo Sin

Assassin/Monk
The Protection Prayers skill Mending Touch is a good spell to remove risky conditions.

Example Builds:
 * Build:A/Mo Enchantment Assault

Assassin/Necromancer
Parasitic Bond is a spammable opener for some hex chains.

Example builds:
 * Build:A/N Repeating Moebius

Assassin/Paragon
Rememdy Signet can be used to cure conditions.

Assassin/Dervish
In the same fashion as the A/R Critical Barrager, the Assassin Dervish could use his skills to greatly improve Critical Hit chance with his scythe. Despite having a huge damage range compared to other weapons, the Assassin's Critical Strikes attribute can increase the chance of critical hits occuring, and when coupled with the Scythe's maximum damage, can result in massive simultaneous damage while gaining energy.

Example Builds:
 * Build:A/D Disciple of Death