If I render my scene using Cycles and have the output file format as PNG there appears to be some sort of tone mapping applied. Setting the render output format as EXR produces linear, non-tone mapped output (as expected).
What is this default tone mapping applied to the linear data output from Cycles before Blender saves out the rendered image as PNG? There's no compositing set up or anything like that.
The display transform for output files is determined in Color Management section. By default, blender uses a sRGB transform, but it can be changed.
(read: Render with a wider dynamic range in cycles to produce photorealistic looking images)
EXRs are an exception and are not processed using such display transform, so they store scene referred (linear float) data.
To bypass the display transform when saving on a display referred format, use the UV/Image editor, and uncheck "save as render"
See this post for more info: What does Save As Render do to the image when saving it?