i notice when i log in with a steam account to the forums, it shows my steam ID number- can i use this to log into my account on a non-steam version of the game, like if i'm at a different computer and they have spiral knights but not steam? sorry if this is in the wrong place.
Not directly since it doesn't use your Steam password. What you can do is login and use the weblauncher through the PLAY button at the top of the page. The game recognizes the launch method and is able to pull your login through your web session, and logs you in automatically similar to how the Steam login works.
However, several functions in the game don't work correctly without the steam overlay. The Energy market page and steam related prompts (like friend invites) will cause the client to lock up hard, and achievements you obtain may not sync with steam after the fact. I haven't tried yet, but voice comm probably won't work either.