Unrooted: only with Android SDK.
Rooted (including custom ROMs of course): install a screenshot app.
Some custom ROMs like MIUI and Cyanogenmod have built-in screenshotting tools. MIUI users can press the Menu button AND Volume Down button simultaneously to take a screenshot. On Cyanogenmod, press and hold the lock/power button; the Power off/Reboot menu includes an option to take a screenshot.
You could definatly whip out that digital camera and snap a quick pic if you're not rooted. I think I read of an app that let's you take screenshots without root. Lemme do some digging, ill post back if I find anything.