Those So-called UV "Protective" Filters
From time to time, I come across someone recommending the use of UV filters on every lens for protection. The theory is that they are cheap insurance to prevent the front element of the lens from get scratched. In reality, I have been shooting for a long time now and never needed this sort of insurance.
I have dropped a lens that resulted in it needing repairs, but I would have required a completely-enclosing filter made of at least several layers of bubble wrap to have made much of any difference. But then I wouldn't have seen much through the lens before it fell either.
And that's my point: having these filters on your lenses will give you an extra layer of glass to shoot through, which will lessen the contrast of your resulting images and potentially cause flare. At the same time, they provide little actual insurance for the dangers that may confront your lenses in the real world. Lens hoods do much better at preventing flare and actually do provide some shock absorption if the lens gets dropped.
If it's scratches you're worried about, glass is actually one of the harder substances around and does not scratch easily. When not shooting, a lens cap works great for preventing scratches.
One thing these filters do do, however, is to keep the filter manufacturers rich at your expense. For me at least, I can do without.
Now if you are shooting around salt water or in a sand storm, a protective filter may be just the ticket, but at least know why you are using it. Don't just use a protective filter because you think you are supposed to.
For an excellent resource on filters, check out Robert Monaghan's Photographic Filter page