I have Photoshop Express, which I think is free and has basic retouching functions and framing/border creations. The noise reduction addon (not free) seems amazing.
This weekend I bumped into Photoforge2 (1.59€) which has a farily complete set of tools, including the ability of working with layers! I played around with it and it really fits in with what I normally do (curves, levels, b&w conversion, unsharp mask...). From the looks and the little I got to play around with it, I think it's worth checking out. Here's a link to the site:
Filterstorm (2 versions, up to $15)
Photogene ($6 I think)
PS Express (free)
If you are looking for a full Photoshop replacement, in my own trials, the $15 version of Filterstorm (Pro) will be the closest. I have not used Snapseed. However, Snapseed is NOT the most expensive.
I have also heard rumors of an iPad version of Aperture is in development. FWIW.