I bought Giants not so long ago. It's not that long a game but damn good fun. Particularly the Delphi levels (and not just because of the boobies).

Yes, Giants or Sacrifice. Go buy :-D
Read a bit about each game first, to find out which would suit you best right now.

Arx Fatalis if you are in the mood for an RPG with atmosphere and a unique magic system.

Seven kingdoms :D!!!

I'm thinking I might get Stronghold actually... I have Stronghold 2 and I've only briefly played the original...

Arx Fatalis?
Days crawling underground. You have to like spiders, though.

What about Fallout? it's still awesome.

Only if you played it first time round. As someone who never played the first one, I found it really difficult to enjoy. The interface is a chore to use :\.

If you're looking for graphic adventures, get Simon the Sorcerer 1 & 2 and the Broken Sword series.
Maybe you want a platformer? Get Oddworld or Earthworm Jim.
If you fancy like putting 30 centimeters spikes of wood into big ass demons, get Painkiller.
If you want to hone your shooter skills, get Unreal Tournament and do some instagib deathmatches.
These are the first games that came to my mind for a reccomendation :-)
EDIT: I just noticed that you asked for games that cost 5.99! Sorry man, I must be very tired...
Well...save my reccomendentions for when you have 9.99 and you'll have some good games to play :-P

You kinda forgot about the $5.99 price limit :).

There is always Waxworks. A really old GOG, I'm sure it needs all the sales it can get XD.

Yeah, I just edited my post :-)
It's just that I'm so goddamn tired! I've returned from work just half an hour ago (it's 10 pm here)....and i can barely put two thoughts together :-P I'd better go get some sleep...

I actually found Waxworks to be a pretty good game.

What about a Simon the Sorcerer game? Three to choose from and each is only $5.99 IIRC.