There is a Teamwork Benefit in Forge of War that gives +1 to threat range if a member of your team successfully crits. It's only against the guy they critted, but its a nice +1.
There are also Shatter Bolts from Dragon Compendium, which are 18-20/x3.
Curse of Arrow Attraction, a Ranger 2/Cleric 3/Sorcerer 3/Wizard 3 spell, gives your enemy -5 to their AC, and makes ranged attacks auto-confirm.
Deepwood Sniper makes all your arrows automatically Keen, and gives you a +2 to the final Crit multiplier of any arrows that you use. This means that you can hit x6 easily, which Dragondoom can translate to x11 against Dragons/Dragonblooded creatures. Now that is bringing the hurt.
Part of me wants to build a Ranger that dips Targeteer fighter, goes into Deepwood Sniper for 7 levels, and is optimized to one-shot dragons.