Photogene² - A mini Photoshop on your phone where you can crop, rotate, edit, add text, make collages, etc. Among the editing apps I have tested, I think Photogene has everything you need. It may take a while until you discover all the features, but once you learn them, it's very simple. 
VSCO Cam - This app also has some of the editing features I mentioned above, but not as good. I only use the app because of the filters that I now often choose instead of Instagram's filters. I mostly use filter 04 and 05.  
PicFrame - Although it's possible to do it in Photogene, PicFrame has many more options when it comes to making collages.
Pixlromatic - I use this one very rarely, only if I'm going to add some extra effects like sparkling lights and such. It can be fun to use at party pictures for example. Added some of these effects on my New Year's pictures.
Afterlight - Bought this one recently and have not had time to use it properly but from what I have seen, it has many nice filters and lighting effects. Could be a new favorite, it's very easy to use and I like that you can adjust the opacity of each filter and effect (0-100).
Instagram - No need to comment.