I use photoshop express mainly to sharpen and straighten, sometimes adjust exposure on the fly.
I use camera plus as my main camera app - just like it as it's all little better overall than the stock app.
if I'm feeling really fancy I use 645 Pro which is the closest app I have found to meld the iPhone and DSLR.
Retouch is great for removing blemishes but does reduce overall quality as most apps do.
Groupshot is pretty cool - you can replace a single person's face/head from a group shot with his/her face from another photo in the series - ever take a few group shots where there is always someone looking away/eyes closed?
for a lot of laughs I occasionally use PhotoFunia - put photos you take onto billboards, museums, posters, etc.