Indoor Walls which don't reach the ceiling: Eruvin 72a-b, 79a-b
For an entity to be part of a wall, one must decide to leave it there: Shabbat 99b
"Combination" Walls
For the height of a wall to include an area which is the wall of a pit, as well as above ground: Shabbat 99a-b; Eruvin 78a, 93a-b, 99b; Gittin 15b
"Imaginary" Walls
Using horizontal ropes, with small gaps in between them: Eruvin 16b
Using vertical reeds, with small gaps in between them: Eruvin 15a, 16b, 19b
Considering a stone as though it were hollowed out, such that it becomes like a corner enclosure for a well: Eruvin 19b
Making a hole in a broad beam which overhangs water, and considering the sides of the beam as walls descending to the water, to permit a person to draw water through that hole: Eruvin 86b-87a
Imagining that part of a wall is hollowed out [Chokikin leHashlim] so that the empty space it surrounds will be large enough to reach a given size: Shabbat 100a; Eruvin 11b, 33b, 101b
Invoking the principle of "Pi Tikrah" to extend an imaginary wall downward from the edge of a roof: Eruvin 25a-b, 90a, 94b-95a; Succah 18b-19a
Using the principle of "Pi Tikrah" to extend an imaginary wall downward from the edge of a roof, to create one or more walls for a house, if the walls collapse on Shabbat: Eruvin 94b-95a
Using the principle of "Pi Tikrah" for walls which meet at a corner: Eruvin 94b
Less-Obvious Types of Walls
A mattress thrown into a pit, dividing the pit as it lands: Shabbat 100a
The sages' leniencies in dealing with walls over water: Eruvin 86a-86b, 86b-87a