Gil Zussman's Cube-O-Rama: OBL:
A very full-featured OBL trainer, with integrations into Dylan's video series above, along with case filtering, random scrambles, and timing.
Shellie's Sillie Square-1 Method:
Although other beginner methods might be faster, Shelley Chang's tutorial from 2009 is quite likely the simplest that exists.
Intermediate Vandenbergh Resources
Lars Vandenbergh's original tutorial:
Although the algorithms are outdated by today's standards, Lars' method is the CFOP of the Square-1 world.
Samuel Fang's Intermediate Cubeshape Video:
Also known as "scallop-kite" cubeshape, this intermediate method adds one more "base case" (namely, scallop-kite) to dramatically shorten many intuitive solutions.
Optimal Cubeshape Chart:
Based on Sam's video above, Reddit user r/olimo created this chart to show how the various cases connect to each other - like with OBL, optimal cubeshape forms a tree :)
Gil Zussman's Cube-O-Rama: CS:
A very full-featured CS trainer, with integrations into Sam's video, along with case filtering, random scrambles, and timing.
Samuel Fang's CP Video:
Good algs for every CP case; many of these will be very useful for PBL later.
Samuel Fang's Intuitive Non-Parity EP Video:
Instead of handling all 49 non-parity EPs as entirely disjoint algs, Sam walks through how to intuitively solve nearly all cases by just using a couple core algs.
Daniel Karnaukh's Non-Parity EP Alg Sheet:
Still, some NPEP cases are significantly faster than an intuitive alg might be; Daniel's sheet contains a well-optimized alg for every case, often from multiple angles.
Advanced Vandenbergh Resources
Eva Kato's CSP guide:
A very well-presented website (with accompanying video) on learning CSP.
Samuel Fang's PBL Guide:
One of the first full PBL resources created, Sam shows how to solve all non-parity PBL's efficiently (most commonly with 2 short algs), along with a slew of tips and tricks along the way.