Google Nik Collection is a set of six powerful plugins for Photoshop, Lightroom and Aperture (they can also be used standalone or with other Photoshop plugin-supporting apps), formerly priced at $149 but now available for free.
Analog Efex Pro enables creating stylised effects exploring "the look and feel of classic cameras, films and lenses".
Color Efex Pro provides 50+ filters for correcting colours, retouching and applying creative effects.
Silver Efex Pro is all about creating stylish and black shots, with an advanced grain engine, emulation of approaching 20 popular film types, even options like toners and borders.
HDR Efex Pro allows exploring the potential of HDR photography, with one-click presets for beginners and plenty of fine-tuning options for the more experienced.
Sharpener Pro provides advanced control over how your image is sharpened, and can be customised depending on what you're aiming to do with the results (display, inkjet, continuous tone, halftone...).
Dfine lets you adjust contrast and reduce colour noise separately, in some or all or the image, optionally with control points to avoid the need for masks.
Google Nik Collection is a powerful set of image editing tools with a host of interesting features. If you don't have Photoshop, Lightroom or Aperture, you can still use them, though not so conveniently: you'll need to launch the executables manually (check your Program Files\Google folder), and open and save images from each plugin.