Luggage and bags:
Make sure your day pack or backpack is comfortable. It's better to have the weight on your hips than all on your shoulders. If it rains and it gets soaked, anything inside your pack will probably also get wet, so you'll be carrying this as extra weight.
Clothing/Shoes/Weather Gear: It can be more than 10 degrees F cooler at the top of the mountain than at base level. It can also be windy and if it rains, it can make for an uncomfortable trek if you're not dressed appropriately. Dress in layers. Use good hiking boots or hiking shoes that have a good tread on them. Consider taking a second pair of hiking socks (not cotton) and package of insoles, in case it rains.
Toiletries and Medical Supplies: Pack a first aid kit. Consider taking some ASA or ibuprophen, in case you need it. If you're taking the Hunt Trail, there is a "last-chance" outhouse at the Katahdin Stream Falls, but after that, there's nothing. You're expected to carry out with you anything you bring in, so go prepared.
Photo Equipment: Oh yes...you'll kick yourself later if you don't take your camera. The "money shot" is standing beside the sign at Baxter Peak.
Consider taking a pair of hiking poles, or even just one. We each used a ski pole, but though we found them useful below the tree line and on the final assault to the summit, we found it was a bit of a nuisance to carry through the challenging Boulders area, where it got more limited use.
Camping/Beach/Outdoor Gear: We didn't have a problem with mosquitos on the climb, so we didn't need repellant. Had it been sunnier when we did the climb, sunscreen would be a wise thing to take. As it was, we didn't need it this day.
Miscellaneous: Take a couple of energy bars, or make up some Trail Mix (I like the recipe at http://appetizer.allrecipes.com/az/78019.asp). I packed 2 cups of Trail Mix with me, but only ate one. They recommend packing at least 2 litres of water. I did, but drank less than one. On a sunnier and hotter day than when we went, you could get dehydrated faster. It's better to have too much water than not enough...you can always pour some of it out on your descent.
More experienced climbers can do the Hunt Trail in as little as 6 hours (we saw a crazy guy running down the trail...he probably did it in less time), but if you're less experienced like my family is, it took us 14 hours. We each had a flashlight to use during the last 10 minutes of our descent. A number of climbers we saw were using lights strapped around their heads.
-Sturdy Footwear: Hiking boots...sneakers probably aren't going to be an adequate choice. Most trails are rock covered and footing can be difficult at times.
-Wool/Pile shirt and or sweater, hat, extra socks and raingear.
It can get blustery at the top.
Toiletries and Medical Supplies: .
-Band-Aids, Ace Bandages, mole skins.
Likelihood of Blister= High
Photo Equipment: .
-Lightweight digital would be nice. Larger formats are difficult to hump.
-Water...err... Did I mention water? Yeah...Water.
At Least 2 quarts when climbing any of the mountains. Consider looking into "Camelbaks".
-Flashlight: "I know you plan to be back in camp before it gets dark but you may still be dragging your *** when the light is fading."
-Snacks: High Energy Bars, candy, nuts, dried fruit, rack of lamb. (---Just testing to see if you are still with me.)
-Map/Guide Book: Know your route; plan for alternatives if the weather gets nasty.
-Other Stuff to Consider Bringing: Compass, lighter, emergency blanket (foil), whistle, knife, voluptuous traveling companion.