For an Ubuntu user interested in sending only one file at a time between PCs, NitroShare is a great file sharing solution. Unlike the options listed above, NitroShare allows you to share individual files, not directories, with other PCs. So if you're needing to share a mounted directory between two PCs, this wouldn't be the option for you. This tool would more closely resemble someone handing you a single item whereas Samba, NFS and SSHFS are closer to using a shared container.
The NitroShare application is available for Ubuntu (and other Linux distros), OS X and Windows. Unlike any of the directory sharing options listed above, this is simply an application which requires next to no configuration at all. It is a good choice if you only wish to share specific items between PCs and have no desire to network those same PCs together.
Ubuntu One is a cloud-based storage solution provided for Ubuntu users and supported by Canonical. It offers Ubuntu users a means of sharing files and even directories over a local network, in addition to making the same content available off-site as well. Available as both a limited-capacity free option or a higher-capacity paid solution, Ubuntu One is quite useful for those looking to keep PC content available in the cloud.
The last option I want to mention is a cloud-based file sharing service called Dropbox. Similar to Ubuntu One, Dropbox differs in that you can use it on any Linux distribution, OS X or Windows.
Like Ubuntu One, Dropbox gives users a finite amount of storage. Should the need for more storage space arise, you can always expand how much Dropbox will handle by purchasing more space. The only real difference between Dropbox and Ubuntu One from a file sharing standpoint is that Dropbox is genuinely, cross platform where Ubuntu One isn't quite there just yet.
Now that I've laid out ample options, I'll conclude this article with a guide to help you choose the Ubuntu file sharing solution that's going to work best for you.
Free and do it yourself: NFS, Samba and SSHFS are you best choices. If speed matters to you, you might want to consider NFS or Samba.
Paid and easy: Dropbox is the best option without question. While there are other solutions out there similar to Dropbox, I've found that when I need directories to sync between computers without hassles or errors, Dropbox is the way to go. If you use Ubuntu-based systems exclusively, then perhaps Ubuntu One is something to consider. If you're simply looking to share individual files occasionally between PCs, then the last option would be to consider NitroShare.