Hammer

I would like to know, is Hammer similar throughout the Valve games?

I might try to learn how to use Hammer to make maps for CSS, but since noone plays CSS that much anymore, this is more of a learning experience until I get TF2 of something like that.

I want to know if any skills that I develope for CSS would translate to TF2 (or other Valve games for that matter).

it's all the same aside from props, textures, and entities associated with each game.

Nobody plays CSS?

O_o

I saw 10 servers when I played earlier today that had people in them.

You must have some bizarre filters on.

I only play on maps that start with "c".

uhh CSS is the most played pc game. it gets up to 100k people daily and a lot more on weekends.

They must be invisible then, because even without filters I only see at tops less people then Halo PC has.

Of course maybe they are invisible, they play with bots, or they are playing Mods that require CSS.

No, something on your end is screwed up.
Just today, at least 3,000,000 minutes were played by xfire users alone...

But yes, to answer your question, hammer is used by all source based games for level editing and anything you learn in it will apply to other source games. Props/textures/entities may be different from game to game but the editing tools are the same for each game.