Thanks to Reddit user A13x231, all 25 prerelease codes for Destiny have been compiled in one, well-organized place.
You can find the codes, as well as a clever Javascript for automatic entry (seriously, how fucking cool is that?) here.
I can verify that the codes and script are both 100% legit.
Pro tip: hit “enter” after you paste the code in to start the script… which you should only do after you log in.
Happy code wrangling!