Hi, I might have exactly what you're looking for. After many months of work, I released this just yesterday for testing: http://www.weighmygear.com/
As you'll see, it's an online pack-weight calculator based upon a checklist system. On top of that, it's connected to a fairly large database (over 6000 items) of weight values for some of the most popular outdoor gear on the market. So, all you need to do is click on items in the lists, and they'll be added to your virtual pack. Of course, the collective weight gets added automatically. Now, if you have some gear that's not in the database, no problem, you CAN enter your own gear. Just type in its name, size (optional) and weight, click on the arrow button (next to the weight) and it will be added to your pack.
This tool is 100% free, and you do not need to register to use the basic pack-weight calculator. However, if you do choose to register, you'll get some extra features, including the ability to save up to 5 different gear lists, print your gear lists, and share your lists via email.
Please feel free to try it out. I noticed in your original post, though, you're doing a Boundary Waters trip, and this app does not currently have paddling equipment, nor does it have spaces in the checklists for it. So, I'm afraid, for this particular trip of yours, it may not completely meet your needs, but in general, I hope you find it to be helpful.
I'd be very interested in your (and everyone's feedback) if you'd like to provide it.