After a few years using Powershot cameras I finally took the dive and bought my first DSRL: a Rebel XTi. Nice camera, I am having fun playing with it.

So I dug deep into my closet and pulled out my old camera equipment from my high school phot classes and surprisingly I still have some Hoya filters in good shape, three of which are the same size as the kit lens on the Rebel: 58mm. The filters I have are 80A, 81C and FL-DAY. I understand that most of what these filters do can be done digitally, especially with the RAW files, but is there any validity to using filters on digital cameras? I think a skylight filter would be mandatory to protect the lens and I am looking forward to buying one soon, also maybe a polarizer filter (I used one a lot back in my high school days). Besides those, what other filters are valid for digital use?

Welcome to the Forum. I think a good circular polarizer is a must have. As for the others, you're right, they can all be done in post processing. It all depends on where you want to spend your time. I like to try to get things as right as I can in camera, but filters add extra weight to your bag, and extra time to use, so there is a trade off. It all depends on what you're shooting... A ND filter can come in handy when you want a slower shutterspeed on a bright day. A graduated ND filter can be handy in contrasty situations, but this affect can be recreated in post processing too.

The skylight filter as a lens protector is debateable. Some people never leave home without them, others don't use them at all. They have saved some people some expensive repair costs, and they have also caused others to miss shots because the extra glass is more prone to lens flare... Personally, I don't use them. If I were going to the desert, or beach, with lots of blowing sand, or saltwater, then I would consider using a filter as a little added insurance policy, but for my day to day shooting, I don't use one.

Agreed - I use a hard plastic (or metal) lens hood all the time and that will give you some physical protection as well as blocking flare instead of adding it. The best hoods are the ones made by the same manufacturer as the lens, specifically for that lens.

I use a UV occasionally if I'm pretty sure the lens is going to get dirty - like in the rain, etc. A UV filter is clear and a Skylight or 1A has a slight tint to it. If you get one, I'd get the UV but again I rarely use mine.

Polarizers are good - and can be used as an ND filter. They're not the greatest for this because you might get some polarization even if you don't want it, and the amount of light you'll block won't be much. Probably about a stop, maybe less in conditions where you're not getting polarization (cloudy day, etc).

I believe the 80A is a tungsten balancing filter. One of the real benefits of digital is white balance. That feature means you'll never have to use a filter for color balancing again. I agree that a polarizer is a must. I use mine for adding contrast and color saturation as well as for reducing light on a bright day. A polarizer can be used in place of a ND filter.

There are lots of opinions on protective filters. I generally use them. Some photojournalists use them instead of lens caps so that they can always be ready to shoot. Other photographers believe they degrade image quality too mcuh and don't use them. I usually buy the best ones I can get (usually B+W). That way at least I know I'm not putting crappy glass in front of good glass.