Presume you mean post-photo editing? Snapseed and PS Touch.
Taking photos... Not required on iPad2 as the camera isn't fab, but with the iPhone there are soooo many apps that it may depend one what result you want...
Instagram requires a login and it posts to the web instantly (but has great, instant filters).
Fotoyaki is a good alternative to the above, IMHO.
Pro HDR if HDR is your thing.
Cinemagram is a cool app to take an image that includes a 2 second video (like a gif).
Sooooo many more! Check more out on the AppAdvice website - under 'applists'