Java Edition:Glass
Glass was implemented in Classic, however it could not be created by smelting sand until Indev. Until then, the only ways to obtain it were from the Classic inventory or the Indev item chests. The discontinuation of Sand during early Infdev made it impossible to obtain through normal means until Infdev 20100413, when sand was re-implemented in the world generator, and thus glass was recontinued.
Obtaining
Classic
From Classic 0.0.19a to Classic 0.30c Glass was available in the Classic inventory.
Mining
From Classic 0.0.19a to Indev 20100207 Glass could be obtained by mining the block, and did not require any silk touch.
Indev Item Chests
From Indev 20100124-1 to Indev 20100128 new worlds would generate with the Indev House, and inside the house were debug chests. Between Indev 20100124 and Indev 20100125 the chests held items in stacks of 99, while in Indev 20100128 the chests held items in stacks of 100. Glass was one of the such items that generated inside the debug chests.
Smelting
From Indev 20100201-1 to Indev 20100218, dropping Sand or Gravel into fire, or lighting a Sand or Gravel item on fire, would result in the item smelting into 1-3 Glass. From Indev 20100219 to 1.21 (present) smelting Sand in a furnace results in Glass.
Silk Touch
With the introduction of Silk Touch in Beta 1.9 Pre-Release 4 until 1.21 (present), when mining Glass with a Silk Touch tool the block would drop itself instead of dropping nothing. This does not work in 12w32a (1.4.2 Snapshot) due to a bug, causing this method to be discontinued for that snapshot.