Key stash or how to store bunch of steam activation keys?

yup...notepad

Used to use Notepad, but for copying titles to trade sites, was much easier with Excel.

Depends how many you have, notepad is fine for small groups but get you can get to a point where it's too cluttered. If you have a whole heap I'd say excel/spreadsheet yes (you can even use Google Docs).

OR you could have several notepad files, but honestly just get a spreadsheet.

Excel, so then you can color the cells to mark whether you've used them.

I used to use notepad, then when I got past like 50 bundles, I switched over to excel

If your email provider has a notepad feature, you can access your list(s) anytime/anywhere you access your email.

Make a software using VB xD

I could actually do that, given enough incentive.

i use excel
you can sort, color or even make some formulas (which game i have and game i make gibs etc)

Nothing, I'm not even kidding, I just leave them in the bundle site and the only list that exist is the one in my trade thread.
That said, it never went over 50 items at a time so is easy to manage them by just memory.

Excel or Notepad or... Make a Notepad empty with the name like "Pokemon Adventure [1115-1MAN-OT3P-4DTX]" after this other, and other, after make a folder and add all this .txt maked there.

OneNote

excel or keypass

notebook.

Lastpass Securenote. Always available online, protected by password.