Mods

I find Hard Mode to be the best.

try eternal silence! oh wait... nobody plays it anymore and the servers are empty ;_:

You mean Mods? Minerva for Hl2.

Grey for HL2.

Pick up S.T.A.L.K.E.R Call of Pripyat and install the Atmosfear mod.

Shit gets intense.

Underhell is a really good Hl2 Mod, its still being updated too! Its made by the same people of Nightmare House 2.
I love both of those games!

Oh, that kind of mods...

try Overwatch and Firearms: Source, one of the best mods indeed!

Yeah I had found one nice like nazi zombies game mode for hl2 http://www.moddb.com/mods/zombie-survival3 i was playing ver 3.0 but there is new one

play dear esther the mod, rather than the game.

Sven Co-op for Half-Life 1 is amazing. Best co-op game/mod I've ever played. Up to 32 players I believe but recently only seen servers go up to 12-24 players. (Oh you don't have Half-Life 1.)
Obsidian Conflict is great too, it's way better than Synergy, although there are some Harvest Moon like maps that I do not like. I don't want to spend the next 30-60 minutes collecting flowers only to sell them for stuff I won't even need in the map.

I've played a really long selection of mods for half life, and a good amount for hl2.
Though i don't recall most of their names, i'll give you 2 names for each game though:

-Master Sword Continued for half life, Multiplayer support RPG mod (single player is dull), it's mostly dead now due to lack of updates but it keeps being revived like 3-4 times a year with something new...

-Dayhard for half life 2, single player only mod, it's mostly for the lols plus some of the soundtracks are fun in that mod.

I'm not sure how, but this thread ended up leading me to The Stanley Parable. It was most entertaining, I highly recommend downloading it.

u can start with those mentioned here: http://www.youtube.com/watch?v=Wu7ekgo0-mk then check out planetphillip.com