I have recently begun to make much broader use of filters in my work. Even with a digital camera, filters are a powerful tool that can greatly expand the range of things you can do with a camera. Here are some of the filters I use

1. Multicoated Circular Polarizer
   • I use this most of the time for landscape photography. It helps mitigate highlights and bring out detail. Great for improving shots involving water, and can help balance contrast a bit when a bright sky is in the frame.
2. Graduated ND filter
   • Essential when you are taking photographs with a high range of contrast. This is particularly common during sunrise/sunset hours, where the sky can be more than half a dozen stops brighter than the landscape. I try to keep a .6 (2 stop) and .9 (3 stop) soft and hard ND grad with me at all times.
3. ND filters
   • Solid ND filters are excellent when you wish to lengthen your exposure time. Many shots can benefit from a reduction in detail in certain things. This includes the noisy ripples on the surface of a lake, or the puffy detail in clouds. Stacking a few ND filters (or using a high-stop filter like the Lee 10-stop 'Big Stopper') can allow you to greatly lengthen your exposure times, and smooth out noisy detail that is detracting from your shot.
4. If you are a DSLR user, many modern digital sensors are sensitive to infrared light. IR photography has become a growing field of photography. An IR filter is another useful tool if you wish to do IR photography, as it filters out all visible light (it literally looks solid black to the human eye), while letting in only infrared wavelengths.

As a digital SLR user, I have not had much need for warming or cooling filters, as white balance can easily be adjusted in post processing. However, if you are an SLR user, warming (and possibly cooling...may depend on your personal style) filters are also an excellent tool to have in ones photography toolbelt. I highly recommend looking into Lee Filters...excellent quality, and one set of filters can be used on a wide range of lenses with their custom mounts.