Ladders

Most Source games feature some mechanism that allows players to climb and descend from ladders.

Left 4 Dead series, Day of Defeat: Source, and Counter-Strike: Source
Use.

Half-Life 2 series and Portal
Use.

Counter-Strike: Global Offensive
Make a regular brush and texture at least one side with.

Portal 2
Ladders cannot be made for single maps, but mods can edit  to make ladders similar to CS:GO's.

Alien Swarm and Team Fortress 2
Ladders cannot be created for these games. Alien Swarm's code is publicly available, so ladder functionality could be added. One potential workaround is creating a very thin and tall staircase of clip brushes.