Can you dial 911 on a landline without service?

If it doesn't have a dial tone, I don't think it will work.

yes you can
in the US most phones have a built-in emergency .
so as long you have a phone that has some kind of connection (with or without service) you'll be able to call 911
personally i have tried with an cell phone without a SIM card and it worked

It does definitely work on cell phones -- however, it is possible for a land line to be *actually* disconnected. (Not a bad idea to keep an old charged cell phone around as a backup to your landline)

If it doesn't have a dialtone, it likely is not connected to anything on the other end..

i think on payphones you can dial 911 without putting in change

Agree with everyone else and would like to add that you don't have to speak to anyone. If you are in a very dangerous situation where someone may try to prevent you from calling (which is a felony in itself in most states) just leave the phone off the hook after dailing 911 and hide or go to another room so the attacker doesnt know you dailed it (you can teach children to do this as well). The police are required to answer the call.

Note on last reply from me: this only applies to land lines which display the callers location automatically at dispath.