If you've got an old computer lying around somewhere then making an FTP server out of it is a fun and quick project. Grab one of the free common Linux distributions and install it on that box. A basic FTP server is included with the base install. Connect this to your home network with a fixed IP address and set up "applications & gaming" on your router to point ports 20 & 21 to that address. This will send anyone asking for an FTP connection to that server without interfering with any of your other online tasks. Add a new user to the system and assign a password whenever you need to share a file with someone. Unless it's someone you share with often you may want to disable the user afterward.

Another nifty benefit is that you can easily grow this into a file server for your home network.

It's a lot easier to set up and administer than anything under Windows. It's also a lot more hackproof. Your router will only pass FTP access to the box so nothing else can get in, and the FTP server under Linux is much more robust and secure than the Windows offerings.

Now, all this depends on the outside users knowing how to access your server. If you're on a dynamic connection then you can probably look up your IP address for them to use as it will probably remain the same long enough for the transfer to complete. You can also set up a free account at some place like dyndns.com to point a fixed name at your connection even if your IP address changes.