Bored of waiting for HL3 ? So am I, but here's a little something to make the wait easier.
MINERVA: Metastasis is a source based mod set in the Half life universe. Its amazing if you're into HL and it's universe, probably a bit less if you're not (you would definitly rather start by playing HL2)
It has similar atmosphere, mechanics (duh), and immersion than HL2 and it's Episodes. Consider it as a plus or a side quest to the main game, kinda the way Opposing force and Blue Shift are to HL1.
It's creator has worked at Valve so no wonder it is a good game !
I really enjoyed the map and plot. No spoiler, but I can tell you that once again you're gonna have to f*ck with some huge combine installations :D
It's hard to tell when the game events occur compared to City 17's events, I think it's just some Resistance action happening before Gordon's come back.
Only downside is that the game had some weird graphic settings the first time played it, it was really bright and well... quite ugly compared to what I was seing online.
If it happens to you (you can compare with the community page screenshots) add "+mat_dxlevel 90 +mat_hdr_level 2 +mat_hdr_enable +mat_fullbright 0" (no quotes) to your game launch options.
The game is free if you already own HL: E1, so get it !