Okay, I’ll bite: why is a circular polarizing filter useful?
They’re useful mostly for landscape photography. A polarizing filter cuts down on glare/reflection, which means that you often end up with skies that look bluer, cloud formations that are more apparent, you’re able to photograph right through reflective surfaces like water or glass, and some colours may even look a lot purer/brighter. Cool stuff.
Minimal benefits as far as portraits or indoors, and totally useless if you’re playing with macro or low-light photography.
Here’s a neato photo a friend of mine took with her polarizing filter.
Your page is private.
Here is a photo I shot using a polarizer: All sizes | AZ_Clouds | Flickr - Photo Sharing!
As you can see, it accentuates the color and contrast between the sky and the clouds. Maybe even too much.
The polarizers need to be circular, because normal (cheaper) linear polarizers mess up most modern camera’s auto-focusing.
That’s an amazing landscape. 
If he does his own editing of his digital photos, I’ll suggest a digitizing tablet. Much easier to work with than a mouse. They all use a pressure sensitive stylus and work very well. Wacom makes some professional models in the $250 range, and the Bamboo series starts at $100.
Really? That’s odd… she’s never set anything to private as far as I’ve ever seen (in fact, she’s very active in the Flickr community and routinely shares her work, and has even gotten to the Explore page a few times) It’s not my account, though, so I can’t confirm the settings.
Nice landscape, beowulff. I may have to add a polarizer to my must-buy list after all.
The salesman in a camera shop mentioned that name, I take it they’re a quality brand name in the field?
I’ll look into lenses as well, but a flash might be interesting. Dad’s digital camera only has (what I presume to be a pretty small) built in flash, perhaps that might be worth looking into too.
I’ll just be contrary here and suggest that, as a general rule, it is never a good idea to give an expert in any field a gift related to that field, especially if you, the gift giver, are not equally expert in the field. The only exception to the rule is if the intended recipient has let you know, intentionally or otherwise, that there is some specific item he’d like to have, and you’re certain he hasn’t gotten it for himself in the meantime.
Now, I’ll admit that the situation with your father and his photography hobby is not quite the same as trying to buy a stamp for a world-class stamp collector. And I’m sure that many of the ideas people have suggested in this thread would be appreciated.
But you won’t know for sure until you see the look of excitement or disappointment on his face when he opens it. Because you don’t know as much about photography, and your father’s specific interests, as he does.
I’m not criticizing your generous impulse, of course. Just suggesting that there may be other gifts that you can be more certain Dad will love.
Also, as I dig into it more, I’m finding that photography can be a very expensive hobby…
what sort of photos does he take?
Indoor studio stuff?
Outdoor natural light?
Of course a gift card to B&H or Adorama would probably be well received.
Here are some suggestions based on cost and type of photography (which the OP hasn’t identified yet):
[ul]
[li]In the inexpensive realm for landscape photography I would suggest either a Circular Polarizer for his most popular lense size or a graduated neutral density filter or a Cokin “P” series filter and one or more filters. [/li][li]For moderately expensive ($40 to $150) you could supply an extra rechargeable battery for either or both of his cameras or a large capacity memory card (CF, etc) or, if he is into portrait photography, a 5-in-1 reflector kit, or a Photographers Vest (~$60-$80). [/li][li]For expensive (>$150) I would opt for a light meter for portraits, the aforementioned tripod (the Manfretto 055XPROB with or without a ball head such as the 3055) for either portraits or landscape or a new lenses or a studio strobe kit for portraits or the previously mentioned flash.[/li][/ul]
It’s good advice, Dad would most certainly let us know if something wasn’t good enough 
I’ll use the advice here to drop subtle hints first perhaps.
Thanks for the suggestions so far.