There are several plugins that I use regularly and Nik Color Efex is one of them. The latest version is a leap forward as you can now stack filters within the application and then save them as recipes. I know that you can probably create the same effects with curves and layers in Photoshop but Color Efex is much quicker.
The danger with any plugin is that the looks become over used and you spot them popping up in lots of people work, you can get round this with Color Efex by using a mixture of modified filter settings and filter stacks.
The plugin is available as a downloadable trial HERE and it will work in Photoshop, Lightroom and Aperture.
One of the best filters and one I use on most of my images is Pro Contrast, it was in Color Efex Pro 3 but it has been tweaked in the version and works even better.
