I've been banging the Half-Life modding drum pretty hard the last couple weeks, but hey, there's been a lot of good stuff lately! For example, Transmissions. It's a Half-Life 2 mod that gives you a new custom adventure along with a new toy: a version of the gravity gun that lets you fire concussive orbs and perform a rocket jump.
The mod begins with the assumption that By Now You Know What You're Doing. Get ready from the start to solve a few environmental puzzles by moving crates around to act as steps, bust padlocks off doors, provide power to generators using ridiculously oversized plugs, carry objects in your invisible hands to act as shields against bullets, and so forth. It's also not going to hold your hand in terms of difficulty: you'll be faced with zombies and Combine turrets from the outset, and you won't have the standard gravity gun to help you cope.
Before long, however, you'll stumble into an abandoned secret lab where a prototype weapon has been developed, the Zero Point Energy Projector. It looks like a gravity gun, but you can't pick up things with it. Instead, you can fire a concussive orb that can be used to kill enemies, blast physical objects, and shatter obstructions. You can also fire it at the ground while jumping, which launches you into the air. While falling, you can also use it to cushion your landing.
The ZPEP doesn't need ammo, but it is tied to your auxiliary power, like your flashlight, so it does need time to recharge after about a dozen or so blasts. And, it's fun. The rocket jump is mainly used for reaching high spots on the maps, but the concussive orb is great for blasting zombies and soldiers, and for sending physical objects scattering before you.
It's something to keep in mind while you play: is there anything I can blast while I'm in a fight? For example, that cargo container dangling from a magnet? The one that strider is currently advancing under? Hey, maybe that will do something helpful.
The mod is short, but very challenging, and there are several arena-type maps featuring swarms of enemies, so be prepared for some scrambling. You do have access to some of HL2's other weapons, like the SMG, shotgun, and rocket launcher, but it's way more fun to simply try to Zero Point the crap out of everyone.
You can visit the Transmissions website right here, which has download links and instructions.