I've decided that enough is enough. My townmates are being resilient and stubborn; our organization, a complete mess. It's time for me to bite the bullet and get out of this town so I can go Die2Nite elsewhere. How do I kill myself?
If you have a cyanide tablet available to you, you can suicide instantly.
The alternative options are myriad, but slower. The most obvious choice is waiting for nightfall. Just wait outside the city gates. Zombies will happily oblige you.
Note that dying of suicide or outside the gates at night will cause any achievements from the previous day to be lost (it's considered not living through the whole day).
If you're really feeling evil, you could open the gates right before the attack to avoid this (and possibly get the last man standing achievement). EDIT: As LessPop points out, the portal lock prevents this. You could still try your luck, opening the gate right before 23:40 and hoping nobody notices.