Actually, the only two filters I use with digital are the polarizer and UV. UV only for windy, rainy days when I think there's a good chance of getting something on the lens - maybe you remember my standard answer to UV filters...

The polarizer is the only filter you really can't duplicate with Photoshop. I have also gotten deep blue skies without, but not consistently. Polarizers have other applications too, and a friend just let me try out a Gold & Blue polarizer - interesting but maybe not real high on my list of stuff to get. Polarizers can also take reflections out of glass and cut glare from scenic shots - like waterfalls, etc. There's more glare there than you might think! This also helps because it acts as a neutral density filter. I guess an ND might be handy too in some cases if your camera has a slow flash sync and you want to use fill flash in bright sun (this is a function of the camera, nothing to do with digital or film).

Red filter: If you convert your images to B&W with the channel mixer, you have a "red filter" there - actually it's one of the three sliders, the other two being blue and green. You can adjust how much "red filter" you want at the time you convert it. I'd rather do this than use a real red filter when shooting. Plus, you still have the original as a color image if you want it.

Softener: There are many blur filters in Photoshop. I can't really recommend one to replace a camera filter - maybe someone else can. Same idea though - you can control how much blur you want after taking the shot, and unlike a filter, you can have some areas of the shot unaffected by the filter (soften one person's face more than another's, etc).

I used to use CC gel filters with slide film. Glad not to have to deal with those anymore outside on a windy day! Now I just do a custom white balance (or shoot in RAW) and I'm set.