Several photo editing programs offer a 'sharpen' filter, which attempts to do what you require. However using it inevitably adds a level of 'grain' to the photo. (Using 'sharpen edges', rather than just 'sharpen' can sometimes make the 'grain' effect less noticeable).
Using an 'unsharp mask' works in a similar way.
I've got a 'full' version of Photoshop, which includes the relevant filters. The nearest equivalent freebie is Gimp:
http://www.gimp.org/
Gimp includes some 'one click' ways of sharpening an image but, for the best results (as with Photoshop) you need to put a bit of effort in, like this:
http://www.gimp.org/t...als/Smart_Sharpening/
Chris