Anyone want to recommend interesting single player Half-Life 2 / Portal / Portal 2 mods?

Obsidian Conflict is a fun looking co-op mod for Half-Life 2.
Supports at least 6 players as far as I know. :)

Stanley Parable

Minerva
G String
1187
Underhell

Estranged
Research & Development

Came here to recommend The Stanley Parable and Minerva: Metastasis mainly, but they've already been suggested. There was another one called Korsakovia that interested me, but I think it broke with one of the updates to Source and the authors haven't kept up with it. Maybe worth a look if you can get it to work, though.

SMOD Outbreak is one of my favourite Half Life 2 mods. I don't believe it uses any episode content. And here's a link for the download if you're interested.

Do you also take PTI test chambers in portal 2? If so I would recommend you all the maps from Azorae, Giguava, AD_79 and of course Mevious. And if you're done with those, check their favourites. It'll take you a very long to beat all of them, but if you're a portal fan like me it's worth it.

Portal Mod: Portal Prelude

Back in the day, I used to play sourceforts , Minerva, Mistake of Pithagoras, Synergy and others such as age of chivalry.

Here's a list and another

Yay, I can now play the mods that require the Episodes.

Back to this topic to download the ones I skipped. :D

Riot act is good.
The Citizen 1 and 2 are also pretty good.