i don't know if other DMs do this or not, but i limit my players to not just the weight they can carry, but the number and size of items they can carry as well. the reason being, in my world, it makes sense that, "i don't care how strong you are, 10 full plate armors won't fit in your backpack!"
i've set an arbitrary number corresponding to the size of certain things and then another arbitrary number indicating how many items a certain backpack can carry depending on it's material. certain magic infused materials fold space a bit and therefore optimize their carrying capacity, but are much more expensive so it becomes something you earn over time.
my basic rule of thumb is, if you can hold it in the palm of your hand, about the size of an apple, it has s size of 1. and then the number goes up or down from there. so for example a belt pouch i think has a hold capacity of 3 or 4 at the basic material level.
this has a lot of benefit to the game because it forces players to pick and choose those things they really need for the mission and those they leave in storage. if we are hunting a vampire or other undead i try to be fair and go with the theme and not throw in a green dragon with brutal 4 fire breath... darn it, left my resist 5 fire armor back in the ship! this is helpful because i think we can all agree that player turns can get out of hand with time consumption...even when you have your turn all planned out the guy right before you can do something that totally changes the tactics of the battle and so now you have to think of something to do different. with players having 500 items with encounter or daily powers plus their own powers sometimes they have too many options.
it's also good because if you can't carry it, then you might as well not bother arguing over who gets to use it until loot has been divvied up at the end of an adventure. some useful items like torches, sunrods, health potions, etc, players can take advantage of right away. others would take too much time to update power cards or other stats if the item has bonuses so we divvy the loots at the end and then players update their cards on their own time, unless the item is mission specific or necessary, like the sunsword in the ravenloft adventure.
also, anything larger than a handaxe or short sword would have to be slung, not stowed in the pack. i also require a dexterity penalty if you're wearing certain kinds of armor and a backpack, because in reality if you're wearing plate armor you're not gona be wearing a backpack. and if you're wearing plate in the middle ages, you have lesser men carrying your stash, like squires. but i require no penalty if you only wear a lesser pack like a belt pouch or satchel, or an item specific bag like a quiver or purse. it gives a great opportunity for players to take advantage of NPC mercenaries for 2 SP a day. its great because they can be targeted and killed like PCs by monsters. the only downside is certain players that throw a fit at every little thing. my brother argued for 45 min about paying a 15 Sp fine to the constable. rather than use it as an opportunity to role play and have a bit of fun, maybe get into a little trouble, all he could think of was that was 15 Sp out of his imaginary pocket.
anyway, i think i've made the case for why this tool would be pretty useful. i think most players would have an advantage by having something like this at their disposal.
key functions i'd like to see:
item creation database. add name, level, damage (if weapon), bonuses and enhancements, and properties. just like the item database in CB. also the ability to customize these items. and a memo box to add additional details or effects. also power cards for such items that can be printed out at various scales.
multiple uses/charges control. some items, like journey bread, can be eaten from or consumed a number of times before they are completely gone. the ability to not only add how many a player buys at the store but also simply input how many a player uses would be great because that way a player doesn't have to keep doing math. if he uses 2 or 3 or 5 more in a day he can just pop that number in, hit enter, and the program updates the total for him automatically. as this number updates the free space indicator should also be automatically updated.
material creation wizard. each different material given it's properties would add a different "enhancement bonus" to the backpack carrying capacity.
backpack creation wizard. the player could define whether the pack is a belt pouch, quiver, bandolier, satchel, or normal backpack. also the player can define the backpack size and other properties like penalties or bonuses with certain armors.
the ability to print a comprehensive list of all player items and weapons with their stored locations tagged next to the name as well as their associated power cards.
all this can be done easily in microsoft access. i also made an EXCEL spreadsheet that does some of what i've suggested and has conditional formatting that lets the player know if he's reached his limit or over limit on how much he can carry. if anyone would like a copy to try it out just let me know. but i think the case for a multiplatform application with the option to integrate with maptool is much more appealing.
now to the good stuff. how to design this utility. the file that stores the data should be independent of the program itself. by that i mean that its structures should be able to be opened by map tool independently. that way you don't have to open map tool to update your inventory. but if you're playing, and you want to import that data into maptool, you should have the option to select the token, navigate to the data file, and select the file association. bam. now all your inventory is available from maptool. now lets say your playing an internet game. the DM clicks your token, takes a quick look at your inventory, sees what you have and what you need. then he can plan loot accordingly.
but wait, it gets even better. if you program it right, you can click other tokens and give items to other players instantly. i'm sure you can think of a lot of other ways to integrate this idea with an infinite number of possibilities.