I frequently need to send a file here and a file there to other people. I'm using this two services for this.
This first is drop.io for quick and hassle-free uploading and sharing of files. Just go over to drop.io, create a so called "Drop", define (optional) passwords for you and your guests, specify what they are allowed to do (just view or allow them to add files), and the lifetime of your drop. Finished. No account sign-ups, no email confirmations. And no need to tidy up afterwards, as the drops are deleted after the lifetime is expired. It's that easy.
You can subscribe to RSS, email, or twitter updates of your drop, which can be stuffed via web, email, phone, and fax. Nice.
Downside: Your drops are limited to 100MB if you're using the free service. Premium services are available, too.
The second service Dropbox has a slightly different approach. It also provides some sharing mechanisms, but it's primarily focused on providing a local folder that is automagically synchronized between multiple computers. Just add a file here, wait a sec, and it is available on your second computer. Synchronization clients are available for Windows, Linux and OS X.
The files are regular files on your local computer and can therefore be accessed without using awkward browser upload/download forms, although a web interface is also provided.
What's nice is Dropbox's versioning feature: Older revisions of your files are available and can be restored. I wouldn't use it for software development, but this is a nice feature for documents and such.
You get 2GB for free. Bigger dropboxes can be rented.
The data is SSL-encrypted on the way to the remote servers and stored with AES encryption,
What I especially like is dropbox's escheresque 404 message :)