Take a look at the yellow parts which are mounted on the wing of an Embraer ERJ-145. What is it?
Two closer images:
They are called vortilons. They can induce a vortex to the upper surface of the wing at high angles of attack, which allows flying at higher angles of attack before stalling.
Additionally, four vortilons were installed on the lower surface leading edge of the outboard wing panel. Their interaction with the wing sidewash at high angles of attack produce strong vortices that are convected to the upper surface, where they modify the pressure distribution and boundary layer development, postponing flow separation and increasing maximum lift. Their shape and position were defined using the 3D panel program. The combined effect of the leading edge droop and vortilons allowed an improved take-off and landing performance without resorting to more complex variable geometry leading edge devices (such as slats), for a small cruise performance penalty. Figure 22 shows a schematic representation of the ERJ145 droop and vortilon.
They were first used on the Douglas DC9. The original patent shows the vortex on the upper side of the wing:
They are Vortilons, a type of vortex generator. The placement of the vortilon low on the leading edge limits its vortex generation to high angles of attack (where the air meeting it is going over the top of the wing), and at low angles of attack, it's just a little fin sticking out, not creating too much drag (but not none).
They are used to improve aileron control at low speed, and/or stall behaviour where the effects aren't necessary at high speed. They are definitely a "band-aid" added to fix a problem that came up during cert testing. You generally don't add projecting bits like that unless you have to.
As @Bianfable wrote:
They were first used on the Douglas DC-9.
On the Douglas quad-jet DC-8, leading-edge slots are used, compared to the Krueger flaps used by Boeing for its 707:
A DC-8 door-operated slot next to an engine pylon (diecastaircraftforum.com)
When Douglas put those slots on the DC-9 (tail-mounted engines) during development, they failed to produce the desired pitch-down. After investigating, they understood that the lack of engine pylons was the reason; the pylons would limit the span-wise flow. Several iterations later of test-pylons, and they arrived at a mini-pylon that did the trick – which they patented and it became known as a vortilon. The name stands for vortex generating pylon. (The final DC-9 design did not use slots.)
1: Waddington, Terry. Douglas DC-8. World Transport Press, Incorporated, 1999.
2: Shevell, Richard S. "Aerodynamic anomalies-Can CFD prevent or correct them?." Journal of Aircraft 23.8 (1986): 641-649.