This service is one of my favorites. You can do all sorts of stuff with it but I’d like to briefly go over how you can use it to send someone a large file (without completely filling their email inbox). Here is how it works:
You head over to www.dropbox.com and sign up for a free (2 GB) account
Download and install the dropbox program.
Dropbox will create a folder in your My Documents folder called My Dropbox inside that will be a folder called Public.
Copy the file you want to send into the Public file Ex: \My Documents\My Dropbox\Public\MyFile.zip
The Dropbox program you just installed will begin uploading a copy to the internet.
Once it's through uploading you can right click on the file you just added to \My Documents\My Dropbox\Public (Ex: \My Documents\My Dropbox\Public\MyFile.zip) and find the Dropbox entry in the menu then select Copy Public Link
You can then paste the link into an email and the person can start a download from the link.
It is rather a lot of work on your end to setup but once it is setup you can easily send large files just by pasting a link into an email or IM. Note: Dropbox is intended to sync files between several computers I highly recommend checking out how it works. Dropbox has a good explination posted here: https://www.dropbox.com/tour
Another option for online file transfer is jetbytes, for more info check out this post.