FreeABL is my idea of easily achieving auto bed leveling on an  Ender 3 without the need to buy anything, using only the parts that already come with the printer
Most common ABL devices use a servo to move a microswitch into place to perform bed leveling, while my design allows you to manually clip the switch into place to do homing and mesh bed leveling, then you unplug and remove it while the print is in progress. This reuses parts you already have and is much less complex compared to a BLTouch or similar, but still delivers accurate mesh values that are perfectly good for bed leveling.
UPDATE: To get a better idea of how this works, see the following video by user @carlosalexanderdev's where he demonstrates it (using @xchgre's design)https://www.youtube.com/watch?v=pUSaR9r2HHE