Great picks already with The Witcher 1 and Half-Life 1
Crusader King's II is really good, although I myself have yet to pick it up but others here can go into more detail on why.
I haven't played NWN yet either, although it has been highly recommended to me (and I think the best place to get it is probably
GOG.com).
I also haven't played L.A Noire yet either but from what I've seen and read it will more than likely be well worth a measly $5.
Playing Portal was one of the most amazing ~3 hour experiences of my life and I highly recommend getting both Portal 1 & 2, but they will go on sale on Steam for much less than the current prices this summer (late June iirc is when they hold their big summer sale event and they will possibly go on sale before then too). The atmosphere, voice work and dialogue in the games are hilarious and top notch, and the puzzles are a lot of fun.
LOTRO is 'free2play' now so you won't need to spend money on it to play it (although I think there is a limit, I'm not sure exactly, Kan might know more but he has a lifetime sub).
I really enjoyed Trine and (surprise surprise) haven't picked up Trine 2 yet and it was recently on sale so it probably won't go on sale until the summer again. I have not heard much about it since its release, but I have also not heard anything negative about it and plan on picking it up in the summer.
I highly recommend Deus Ex, it is also
$9.99 on GOG.com although the
modding guide (really easy, basically a high res texture pack and a mostly bug fixing mod) works with the Steam version too. The game has very interesting plot based off of various conspiracy theories and a mixture of stealth and action gameplay as well as multiple routes through many levels and interesting details. The only two things I didn't like was the sound of the wind and the guns can be a little awkward due to the age of the game but it was a very rewarding experience.
I've heard great things about Sins but I haven't played it myself since I am not really that interest in that type of RTS.
DA:O is a very enjoyable game, but is not nearly as good as the Baldur's Gate games. Hundreds of unique NPCs to talk to, dozens of which have very involved stories of their own, and a very rich world to explore. There is a "true to the original" major update-thing with added content being made by Beamdog (see
this thread for more info) but it will not be out for a while and no one knows how it will turn out (thankfully Bioware doesn't have anything to do with it anymore). As you have luckily found, you can get it pretty cheap and the 4 disc set is a nice find (they are also available on GOG.com (
BG1 &
BG2).
There are also hundreds of mods and user created additions to the game (which can be a little overwhelming with the
mod guide I followed) but a few mods like a wide screen fix and porting BG1 into the B2 engine were totally worth going through it.
EDIT: Feel free to ask any more specific questions about the games, and what are you looking for in a new purchase the most?
EDIT2: Why delete the OP? There is nothing wrong with asking about various games here :/