You can actually build up a targeted list of prospects that are interested in your products by offering a relevant download on your website. For example, let's take a look at a very good example -- apple.com. When you download the free iTunes and Quicktime software from their site, they will ask you to fill in an optional name and email form so that they can send you offers on songs that you can purchase via -- guess where -- iTunes!
In reality, you do not need to offer such a "heavyweight" download such as a full-feature software like iTunes. You can attract prospects equally well with some quality freebies such as a simple report, a free wallpaper, and so on. The important thing is that your download offers enough value for the prospect to be willing to give away his/her own email address to get it.
However, slapping together a simple download and putting a link on your website won't be enough to attract qualified prospects. You will have to do some homework in order for your lead-generating mechanism to work well for you.
First of all, you must place your download form prominently on your website. Preferably, dedicate a page to it and link to that page from every other page of your website. That way, there is no way your visitors cannot find the download page, and when they do, you'll get some of them converted into your prospects!
Also, you have to put a little effort into promoting your download. Explain and elaborate on the values of the download, and why your visitors should download it. You might think why would anyone want to pass on a freebie, but most of your visitors would be too lazy to take the effort to download it because most of their downloads just sit on the harddisk collecting virtual dust. It is hence important to show your visitors why they should download your freebie.