Research & Development takes the non-combative approach of Portal into the gritty environment of Half-Life 2, and does so with a refreshing amount of original mechanics and crafted special effects. Here's a mod that not only delivers a unique, gun-free experience, but one that effortlessly introduces fresh Source engine effects at surprising regularity. The mod begins with little pomp, the player awakening in a detailed but generic location. A sign on the wall announces that this is a "weapons free zone," while another provides the first puzzle clue.

There won't be as many clues after that, as R&D largely leaves the player to discover solutions in his own time. Many are straightforward, but others are sure to induce some skull-scratching. Nearly all have satisfying conclusions, often complete with humorous scripted sequences involving enemy grunts.

If R&D has one significant fault, it can be found in its own ambitious design. Bugs can creep into the physics-heavy puzzles, and some are made frustratingly confusing when unexpected mechanics are introduced. For instance, you may curse Matt Bortolino for several minutes before you realize that the padlock on the door can be broken by throwing a brick at it. I always just used a crowbar.

But in the end, after solving an obtuse conundrum, you'll likely be praising Bortolino for his attention to detail and uncompromising approach. This isn't a lightweight, by-the-numbers gimmick of a mod, and it's better for it.

Download Research & Devleopment, and stick with it for at least a few stages. You'll be glad you did.

Important note: Research & Development requires Half-Life 2: Episode Two to run. Also, if you're having problems getting the game to show up in your Steam list after installing, try restarting Steam.