Suggest me an online game to pass my time that doesnt require me to download anything

Play spiral knights. Trade items from it.
If you are looking for small games, you are limiting your options.
For a steam game close to the file size that you are looking for, McPixel only takes about 64 MB of storage, I believe, although it is offline.

I would check out 8 bit MMO.

Strikeforce Kitty!!! It's an online flash game. I played on Kongregate (The link). Simple concept, very addicting, replayabe, and a lot of unlockables.

Realm of the mad God, Runescape, Blockland, Spark Rising.

http://tagpro.koalabeast.com/

Capture the flag with other players. Massive community, loads of people always playing. You can find the main community site over at Reddit.

Ratz Instagib (Unity)

Apart from the mentioned games, Board Game Online

Pirates Love Daisies

Hello Kitty Online

kdice. Like risk but better.

The very defenition of playing a game online, involves you downloading something.

Try kingdom of loathing, it is hilarious and strangely addictive. It might seem weird at first but give it a chance, I constant go back and play there is an impressive amount of content for repeat playthroughs.

Pretend You're Xyzzy

I don't see any great <50 mb steam game recommendations..

KingsRoad. An isometric little rpg, quite fun.

Need to waste time without downloading anything? http://www.coolmathgames.com/

For a unity fps game, I highly recommend Contract Wars which is on kongregate and facebook.

There is almost literally no game fitting your description, most require at least something like Flash or Java, if not their own client.
Unless of course you'd like to play the crappy text-based Neopets type games scattered through-out the internet.

Battlestar Galactica online 3d browser based space game as much as I remember there's a 50-300 mb unity download before you can play but that's all

travian

If you just want to pass time, then learn something,.. I suggest you to learn something.

SG metagaming

angry birds

kongegrate newgrounds or zombies ate my pizza

I like Dota 2 and CS:GO too. I would say:
World of Goo (69 MB)
Plants vs Zombies (49 MB)
Binding of Isaac (42 MB)
Super Hexagon (32 MB)
Terraria (63 MB)

I know we've had our differences in the past, but putting that aside in the name of excellent gaming without heavy downloads, check this out. Elements. If you like any sort of trading card game/collectible card game, give this a try. There is some amazing ply depth, collection and crafting mechanics, several tiers of AI to fight, and a decent PvP arena.