Photoshop has a free one that you can use (Photoshop Express). It is limited in what you can do. I don't like it because you have to put the files in your DCIM directory to edit them, I don't. I have mine in a directory called "Pictures" and I have sub-folders for whatever the pictures are (kids, work, Girlfriend, Concert etc.).
The stock one (Gallery --> pick a picture --> once it is displayed, pick menu or pick on the picture --> Edit), works for my needs if I need to edit them
If I really want to edit the picture, I will transfer them to my PC and use Photoshop.
There are some fun photo apps that I use that you can put people on posters, walls, magazines etc. That is called PHOTOFUNIA. There is also Pho.to Lab (yes, it has the period in the name)