Windows/Mac/Linux: Adobe Lightroom might be the professional photographer's photo management tool of choice, but you can save $186 and download free and open source alternative LightZone instead.
The program lets you quickly manipulate a batch of photos, with impressive effects and styles and finely adjustable tools. Instead of layers (a la Adobe), colour masks and masks, LightZone divides an image into 16 "zones". Each zone varies in brightness by 50 per cent from those adjacent to it, and you can easily manipulate the zones to change brightness and contrast. You can change parts of an image by drawing a vector shape. LightZone also can instantly "relight" an image so it appears as if you took it under different lighting conditions.
Overall it's a fantastic tool for making your photos look amazing, and it's completely free. (LightZone used to be a commercial product, but developer Fabio Riccardi recently made the program open source.) To download it, you'll need to register at the LightZone project page and await approval of your account, but the wait isn't longer than 3 hours during US business hours.