Many ways.
The simplest involve variations of just copying your library:
http://manual.calibre-ebook.com/faq....ter-to-another
Which can also be automated with backup/sync software.
Another approach is to use dropbox or a similar setup.
Caveats:
You have to sync an entire library, not just part of it (but you know you can have multiple libraries, right?)
You want to be very careful to restrict your editing/updating to just one machine at a time (or at all), or you can really mess things up.
You don't want to sync cross-platform unless you really know what you're doing (case sensitive/insensitive file structures are particularly problematic).
Having said that, maintaining a library on, for example, a Windows desktop and keeping a "read-only" copy on a Windows netbook is dead simple.