I will gradually add to this thread as I always tend to find new entities I did not know about when I am mapping, or looking for one that I need for a plugin. These can be very useful if used properly, and can really expand what your sourcemod plugin is capable of accomplishing.
I am not positive what games these are in. When I was looking at the developer wiki list, I kept hammer open and varified they were atleast in TF2 as available. This does not mean they will necessarily work (although chances are they do).
If you come across more entities that you think would interest people, just post them here and I will add them to the list. Remember, we cannot create brush entities such as funcs, only point entities, so that does constrain what we are able to do from a plugin.
env_player_surface_trigger: This entity monitors that type of material the player is walking on. You can fire outputs based on when they are standing on a specific type and when they are not.
env_screenoverlay: It has asked and been accepted as common knowledge that you cannot directly draw to a players screen with your plugins, aside from text. This entity allows you to use any texture as a screen overlay to create new effects.
env_shake: Adds a shake to a players screen. Great for giving the feeling that theres an earthquake, explosion, etc happening around them.
env_texturetoggle: An entity that allows you to change the textures used on brush built entities.
phys_ragdollmagnet: An entity that acts like a magnet for ragdolls. Creates exagerated ragdoll behavior.
phys_torgue: An angular thruster, used to apply angular force to an entity.
material_modify_control: An entity that can be used to directly control material vars. To use it, you need to make sure the vmt of the material you intend to work with has the "MaterialModify"
Material Proxy in use. Then, set this entity's parent to a brush or model entity using the material you want to control.
Linear Mode
