Box.net is a great option for delivering your files to clients. The word 'great' actually doesn't even begin to describe how useful it really is - it's fantastic. Basically, you can upload your .zip files containing the photos from your shoot to your client's folder, then give them private access to it.
As you can see from the screenshot above - the sharing options are pretty slick. You can simply copy/paste the link Box.net gives you and email it to your client. Another way to grant people access to the file is by clicking the 'send link' option, which then allows you to enter the email addresses of the people you want send the files to. Box.net immediately sends an email to the people and allows them to download the file. The only downside to this method is the all-unknowing spam filter on the other end. If you have a 'Pro' account (paid), you can password-protect the download links for an added layer of security.
The above screen is what your clients see when they click on the link you send them. After they download the file, Box.net generates an automated email to notify you that the file was downloaded. This lets you know that they got your file and everything worked out.
And finally, storage. Free accounts come with everything mentioned above (sans password protection) and a whopping 1 GB of storage (not being sarcastic). I say whopping because files for MLS systems are so small, it would take you a very long time to even reach half of that - that is if all you are doing is files for MLS. If your clients want their full-resolution images for ads, Box.net may not be the system for you. Free accounts have the 1 GB limit, and they also have maximum upload size restrictions. Each file you upload can only be 25 MB large, so if you have 160 MB of files to get to a client, you're going to have a number of .zip files to upload, and for them to download - making the process a lot less smooth on both ends.
Just to clarify - I talk about .zip files as if they are the only way to deliver files. They aren't. You could get your files to your clients one-by-one if you wanted, but I wouldn't recommend it. I use .zip files because, most obviously, you can compress a folder full of images into one, they can be uploaded and downloaded easily, and they are compatible with both Windows and Mac users.
Storage: 1 GB
Upgrades: $9.95/month for Individual, $15/month for Business