Flare 2 for Mac (currently on sale for US$9.99 regularly $14.99) is a well thought out and executed app for photographers who want to get beyond the basics in photo effects.
The app comes with dozens of filters, and each can be adjusted by the user, making the options almost infinite. In addition to things like the usual color washes, there are textures, borders, frames, tilt-shift, Bokeh rings (out of focus points of light), vintage film effects and more.
RAW formats are supported, along with niceties like unlimited undo and batch processing. Photos can be exported as JPG, PNG and TIF. There's a complete online user guide which is detailed.
Using the app is simple. Import your image, select a filter, then modify to taste. Previews are real time, and the layout of the app encourages experimentation.