Offering Free Downloads on Your Website.
Here's a question: how can you make your website wildly popular overnight? The answer to the question is 'offer free downloads'. Everyone loves to get something for nothing, and downloading is as old as the Internet itself – but it never declines in popularity.
Screensavers and Wallpapers.
Wallpaper is an ideal thing to offer as a download: it's popular, replaced often, and doesn't need you to host prohibitively large files. Unfortunately, the most popular kinds of wallpaper show characters and images that you're unlikely to be able to get a license to use, but, luckily, you can still create good wallpaper using nothing but geometric shapes and interesting colours.
Any artist worth their salt should be able to fire up Photoshop and produce quite a few very appealing wallpaper-sized images for very little money: it's just not that difficult, if you know what you're doing. However, you do need to remember that you'll have to offer each wallpaper in different sizes for different screen resolutions (so one for 800x600, one for 1024x768, and so on), which can be troublesome. Wallpaper should generally be one part of a site rather than the only thing the site does.
The same thing goes for screensavers. Screensavers are harder to produce than wallpapers, but they have the advantage that the user is likely to spend longer looking at them. If you get an artist to create them using Flash, you should have a relatively easy time. Don't be tempted, though, to have screensavers made that simply consist of the same animation looped over and over again.
What makes a good screensaver, then? The answer is that it should be either useful or interesting, and it must be one of these things for longer than five minutes. An ideal screensaver is one that provides useful information from your website that a user is likely to need every day – but, if you're just going for interesting, you can do something as simple as using randomisation and mathematical equations to produce different patterns every time the screensaver is started. For some ideas, take a look at the screensavers that come with Windows.
Demos and Trailers.
Demos of software and games and trailers for films are very popular items on the web, with literally millions of people searching for the latest ones every day. Even better, because they essentially serve as marketing for the companies that produce them, they're typically freely redistributable by anyone who has the bandwidth and the inclination.
In that case, why isn't everyone offering demos and trailers to their visitors? The answer is bandwidth costs. An average trailer or demo can be anywhere from ten megabytes to about fifty – multiply that by thousands of visitors per day, and then see how many gigabytes of transfer you'd need per month. It adds up fast.
How can you solve this problem? Well, you can try to pay for the bandwidth using advertising, but you're unlikely to make a profit that way, unless you bombard the viewer with ads to the point where they'll just want to escape. Realistically, the only way to make a profit on high-bandwidth items is to use the queue ruse: that is, force people who want to download to wait in a queue for a set length of time, and offer them a button that lets them jump the queue for a relatively small amount of money. You'd be surprised just how many people will click that button – the cost of a gigabyte of bandwidth will easily repay itself five times over. Many gaming sites sell monthly subscriptions that get visitors nothing more than downloads of demos, and they do well out of it.
The queue approach will, however, have the effect of reducing your site's popularity, as many people will just leave instead of waiting or paying. This is the paradox of free downloads: offering them out there completely for free will get you thousands upon thousands of visitors, but you'll be losing money on it because of the bandwidth costs. I'll leave this as a problem for you to solve, but I would suggest that you could do well out of it if you had a related business of your own to advertise, instead of just taking a cut of external advertisers' profits.