If you're a Half-Life fan, here's a mod you should play: Entropy: Zero 2.

It's a mod for Half-Life 2, about as long as Episode 2. Mostly uses familiar weapons, but also brings back some beta concepts (and uses them very well). Has good original voice acting and a great story that for now fits with the canon. Very much feels like a Half-Life game.
You can also play it's predecessor, Entropy Zero, but it's not a requirement for the overall story. Entropy Zero is a much less ambitious project than it's sequel, but still fun. Both are available on Steam.

*minor spoilers*
*you were warned*
You play as a combine soldier, so you get quite a different experience, with different enemy types. If you're a HL veteran, you're gonna see the return of a few old alien "friends" that will try to maul you. You're also gonna encounter some basic Portal puzzle elements. Not anything too hard, but if you're completely unfamiliar with it, you might be confused one or two times.

Don't want to give any story spoilers, but I'll add this: do the "gnome" achievement. It's easier than in Episode 2, and it's worth it.