At a time when most ISPs are tightening their data caps, Zen Internet has just announced that it’s increasing the monthly allowance for its subscribers by 5GB, at no extra cost.

That means customers like me who are on the Zen 8000 Active Broadband service will now receive an allowance of 25GB per month.

That’s all smashing and lovely, but the odd bout of Call of Duty 4 on the Xbox 360 aside, I rarely do any significant damage to my data allowance (as you can see from the graph below, click to enlarge). In fact, this morning, midway through the month, I’ve only used 3.8% of my existing 20GB allowance.

Which leads me to wonder… wouldn’t it be great if you could sell your excess capacity to the download hogs who want to be running BitTorrents 24/7? I’ve got 10GB to spare this month – let me sell it to another Zen customer down the road who’s hit the wall and wants to download The Apprentice on the iPlayer tonight. He gets my data allocation at a discount rate and I effectively get a fiver off my monthly bill. Zen could even set up an Ebay-style data auction on its website, and take a cut for its trouble.

I’m not hopelessly naive. I know Zen and other ISPs rely on muppets like me who buy capacity far in excess of their needs. But surely only a small percentage of users would go to the bother of trading their excess data. And it would give me another incentive to stay with an ISP which charges a hefty premium for its excellent service.

In the meantime, if anyone’s got any good ideas what I can do with my spare 20GB, let me know on the comments below.