So. Those are games that don't need to be downloaded. Maybe they're not as long as other games, and they don't have high graphics, but there are true gems hidden in the form of a time killer.
Here are my recommendations:
Page: Nitrome.com.
Easily, the best minigames creators I've seen. If you've never heard of them, it's going to be fun. Some of the best games:
http://www.nitrome.com/games/nitromemustdie/#.U-PZgvl_vTo
This was a special of the 100th game of that page. It consists of you (and one friend, if you want to) going over 100 floors (I'm not going to make any connection with any famous comic here) killing every enemy that shows up (normally from one of the games done by that company) with a huge variety of weapons (normally from a game of that company, like ,for example, shooting penguins).
Difficulty: High (more and more as you go up).
Level of Hookness: Pretty high, especially with a friend.
http://www.nitrome.com/games/badicecream/?game=badicecream#.U-PasPl_vTo
This is the first of three games. You (and again, with a friend if you want to) have to collect every fruit in each level, avoiding strange mutated creatures of your worst nightmares. Oh, and you're a badass ice-cream.
Difficulty: Normal, depending on the game and on the levels.
Level of Hookness: Normal.
http://www.nitrome.com/games/cavechaos/
Two (or one, again multiplayer) strange minecraft hooked lemmings that have to run because the cave they are in is falling. It consists of two games.
Difficulty: I don't remember much of it, but I think it was high.
Level of Hookness: Again, it was some years ago, but if I don't remember bad, I kept playing for two damn hours, so taking that in count, high.
Now, on to another section of Nitrome. Which is Pixel Love.
Some time ago, they decided to do this section, recommending other games of various companies, which they thought were good. There is a page for all of them, but I will recommend one of them.
http://www.pixellovegames.com/games/thevalleyrule/
A strange, beautiful game, that can be done in an hour, two or half (depending on your skills). I feel oddly compelled with that game.
You can see the rest of the pixel love games by scrolling down "The Valley Rule" screen.
Difficulty: As I've said, it depends a lot on your skills. As I've always been bad at games, it was difficult, but others could see it easy.
Level of Hookness: I didn't want to stop playing. High.
http://www.minijuegos.com/juego/meat-boy
The site is in spanish, but the game is english. It's about a piece of meat that has to rescue its girlfriend (or maybe her friend, I don't know). You probably know the full game, but that one is called "Super meat boy" whereas this one is "Meat Boy". There is also an expansion of it, just below the actual game.
Difficulty: Oh My Motherfucking God, Who Did This Shit.
Level of Hookness: Life is not worth it anymore.
So yeah. Recommend yours, so we can kill time.