The only all around (works most places) spell checker available for iOS is the native one that Johanna pointed out. It's up to the apps you use to support it. Some apps will only use spell checker if you turn it on inside that app; like Pages. Other's use the general settings.
I keep a dictionary app on the iPad for those instances that the built in dictionary and spell checker are inadequate. It's not convenient to use, but as iOS has improved I've needed it less and less.