This question exists for 3.5, but I haven't seen it for 4e.
How far can you go with crit range expansion in 4e?
I'm not interested in how far is worth it, or value judgements, just how far it's possible to go with published 4e material. Items, feats and other means are fine, for any class.
Critical expansion is usually only provided with a single source as crit ranges are stated explicitly with all sources I'm aware of.
The most you can do for an always on effect is the Rogue's Dagger Master paragon path which provides 18-20 always on.
There are a few encounter or daily powers that may expand it more for an attack, a round or even a whole combat against a single combatant. The most these seem to expand is 16-20 and the majority stick to 19 or 18.
The highest crit range I can find in 4e is 16-20. The only instance of 16-20 I can find that lasts longer than a round is Anthem of Progress, a level 26 daily power from the Chosen Epic Destiny (requires: any divine class). Anthem of Progress creates a burst 10 targeting the caster and his or her allies; for the rest of the encounter, all targets crit on 16-20 with At-Will attacks.
On the other end of the spectrum, Tome of Lightning can get you a 19-20 crit range for Wizard lightning powers as early as level 1 (level 4+ item), and Battle Staff can do the same for weapon attacks. The Ogremight Bruiser, Ogremight Ravager, Steel Vanguard Captain, Steel Vanguard Soldier, and Steel Vanguard Student feats can all get you a 19-20 critical with certain powers (or while charging, in the case of the first two) at Heroic tier.
The most universally-applicable (in the sense that you can use the most powers with it) source of crit expansion may the Jagged Weapon enchantment (level 12+ item). Jagged Weapon can be applied to axes, heavy blades, and light blades, and gives you a 19-20 crit range for all attacks with the weapon, and your criticals apply ongoing 10 damage (save ends) or ongoing 20 damage (save ends), depending on the level of the item.
Many classes have an Epic tier feat that gives a 19-20 crit range for powers from that class. There are exceptions, of course, but it's a common type of feat.
