If you want to share files with friends or colleagues, forget the floppy disk on your desk. Instead, consider the one on the other end of your modem--one of the new crop of Web sites that act as parking lots for your data.

All of these sites work in the same basic way: They let you upload files, usually up to 25 megabytes' worth, and store them on their servers. You can then allow anybody or just selected users (to whom you've e-mailed the location and password) to download those files. None of this costs a penny to use; in return, you and your pals get to watch banner ads on the site.

Using these sites is easier and quicker than sending around e-mail attachments, learning how to use FTP (file transfer protocol) servers or snail-mailing Zip disks. And using the Web to warehouse your files means your recipients get to choose if and when to download the data, instead of waking up to find a gargantuan file jammed into the e-mail in-box.

The interfaces for these Web tools are generally easy to use, and the registration processes are no more tedious or invasive than signing up for a free e-mail account. Where these sites differ is in their extras:

* X:drive offers an optional utility that lets you manipulate your stored files from within Windows, as if they were on a separate drive on your computer rather than on a Web server.

* I-drive also offers its own special software; these Windows-only tools can archive Web pages into an I-drive folder and automatically synchronize files stored online with those on your computer. In addition, the site lets users back up their bookmarks and "sideload" MP3 audio files from the popular MP3.com site into their I-drive directory.

* Visto offers 15 megs of file storage but also serves up a batch of personal info-management tools, such as a personal calendar and group schedulers, and allows you to synchronize your calendar file and Web bookmarks among multiple computers.

* Driveway offers an incentive package: The more new users you bring to Driveway, the more storage you can get over 25 megs, to the tune of 2 megs per additional person.

* Free Disk Space tops that, giving users 5 megs of additional storage for every friend who signs up.

* Click2Send uses a safe-deposit box metaphor and allows 50 megs of storage per user--but it stores files for only 10 days. Click2Send also requires more registration information--your street address.

Who's best depends on your own uses and needs. Click2Send stores the biggest files, but for long-term storage it's a poor choice. Both I-Drive and Visto offer worthwhile extra functions, but not everybody needs to beam MP3s back and forth or Web-ify their calendar. Note that these sites don't work well for file backups; 25 megs just isn't enough disk space to store most people's personal files.

And because these sites are, in fact, on the Web, embarrassing downtime problems can arise. Visto users, for example, were met with this notice at the site earlier this week: "Dear Visto Member: We are experiencing an unexpected interruption of service and are working diligently to identify and correct the problem. We expect to be back on-line shortly."

At least downtime only happens occasionally. A 56-kbps modem, however, never gets faster. If, like most people, you use one of these, the limit to the utility of any file-storage site won't be its disk space; it will be your own patience.