I got this build from Frizz... dunno if it was his own creation or if he poached it from somewhere else, but credit where credit is due. It's not QUITE as effective as a ranger or mesmer interrupt, (particularly if your crit strikes attribute is too low)but it's darned good fun.

Skills
Critical Eye (Critical Strikes)
Way of the Master (Critical Strikes)
Disrupting Accuracy (Marksmanship)
Quick Shot (Elite, No Attribute)
Keen Arrow (Marksmanship)
Critical Agility (Sunspear)
Critical Defenses (Critical Strikes)
Sunspear Rebirth Signet (Sunspear)

For PvP replace Critical Agility with Assassin's Remedy and Sunspear Rebirth Signet with the standard res signet.

Attributes
Marksmanship 12
Critical Strikes 12 + 1 + (1 - 3)

Equipment
Any max damage bow, a sundering bow of shelter, fortitude or enchanting would be good.

Keen mask (Critical Strikes +1, stacking)
Rune of Critical Strikes (the higher the better, but minor works fine)

Use
Keep Critical Eye, Way of the Master and Disrupting Accuracy up all the time.