Azure Sheep (Decent. Closer to classic HL stuff than the other mods.)
Case Closed (I don't recall liking this that much. More classic stuff.)
Night at the Office (I think this is the title. There's two endings. Both suck. General gameplay and story doesn't have the polish it needed either. I uninstalled it..)
Poke646 (Excellent. Remember to play the Training Level first before the main game.)
They Hunger (Great)
That's all I can remember at the moment. Mostly because half were already recommended. >.>
I recommend playing Poke646 first, followed by the They Hunger trilogy. That should keep you occupied past Christmas (unless you play a LOT in one sitting).
Also, Digital Paintball is MP only. :p
For the officlal stuff, Condition Zero is ok up until Hard difficulty, then it's just annoying, because you'd swear the bots hack. >.>
Condition Zero Deleted Scenes is a ton of fun though. It's closer to the style of Half-Life than CZ, but with CZ weapons and stuff. By that i mean you go though a series of levels and kill enemies with (relative to CZ) ease.
OP4 and BS are, as you know, both good HL mods.
There are some good MP mods with bots that you might find fun. The Specialists is a great mod, the bots are a good difficulty. Easy enough to take out, but they will still sometimes react faster than you.
RCBot is a bot addon for a few MP mods like SvenCoop and Natural Selection. The bots tend to actually work pretty well, except in NS if you play the regular game mode, a human player will be needed on the marine team to serve as commander. Otherwise the aliens will win very quickly. :p And in SvenCoop the bots can only solve simple hit-the-switch-to-accomplish-the-objective puzzles (basically they activate anything near them). Anything more complex needs human players.
RCBot is a slight pain to set up and use (you need to be familiar with HL's console), I recommend the version that does NOT use metamod. The directions to install it should hopefully be clear enough.