Bynamite, Mr. Acquisti said, is “simply trying to make these kinds of transactions explicit, more transparent to the user.”

Last week, Bynamite introduced an early, or beta, version of its software, a downloadable plug-in for browsers. That software and its Web service monitor what ad networks and e-commerce sites collect and assume to know about a user. A user’s interests are then assembled on a Web page, grouped by categories like “news and current events,” “general health,” “travel,” “technology” and “shopping.” The categories are weighted by how often you visit different categories of sites or make purchases at some online merchants.

The information tracked by Bynamite is steadily updated, and, at least for me last week, a small pop-up alert at the bottom of my computer screen appeared every day, informing me of new information about me from ad networks. Mr. Yoon calls the product’s early version mainly a “mirror,” showing users how the commercial Internet sees them.

Users can change that mirror to represent their interests more accurately. For example, I don’t own a car, but my “automotive” folder soon had several entries, saying I was interested in Mercedes-Benz and other brands, presumably because middle-age men who visit the Web sites I do are typically attractive targets for car ads. I deleted the auto interests, suggesting to advertisers that I’m not necessarily a good prospect. Still, I saw a few car ads on sites I later visited.

Bynamite is by no means anti-advertising. It does not block ads. Its Web site recommends free tools, like AdBlock and NoScript, for people who want ads blocked.

In essence, the company has a libertarian, free-market ethos. If consumers have more power and control, it says, personal information should flow more efficiently to the benefit of both consumers and advertisers, who will be able to more accurately aim their ads.

Like most start-ups, Bynamite faces long odds. To succeed, it must be easy to use, and users must trust it as a reliable middleman handling their data. It has no business model yet, though it could offer product recommendations, based on interests, and collect fees on resulting sales from merchants. It hasn’t ruled out accepting ads itself. To start, its free plug-in software works only on Mozilla and Chrome browsers.