So, if the problem with the original Portal was that there wasn’t enough of it, the solution is probably Rexaura, a new mod for Portal 1 (Which it requires). It’s a new set of test chambers in the original universe, based around extended mechanics for energy balls, including one-shot redirects, balls that explode if you stop holding the button and energy gate switches they have to pass though.

Rexaura does a very good job of introducing you to its new concepts, all of which seem logically thought through (Caveat: I’ve not played though the entire set yet) and while the writing falls short of the original, it’s better than most games. If there’s a complaint, it’s that it tends slightly further towards the timing-based shoot-portals-quickly mechanics that Valve themselves steered away from for the sequel, but it’s still an entertaining series of new and original Portal levels, and well worth the nothing you’re paying for it.