I rely on a combination of computers and devices at work, at home, and on the road. My collection consists of a Mac Pro at work and an iMac at home, both of which run Parallels, combining the Mac OS and Windows XP. I also have a Sony Vaio laptop that runs Windows XP, an iPhone, Apple TV, and multiple iPods.
With all of these various computers and devices, the chance that I would want to play a song on my iPhone that was only on my Apple TV, or the chance that I would need a file at home that's only on my work Mac is very high. I hated the idea that I would need access to a document, PDF, track, or movie, and not be able to have it locally, regardless of where I was at the time.
After much trial and error, I have found what I believe to be the ideal solutions for syncing my documents, photos, music, email calendars and contacts.
Syncing Documents and Photos
I've tried them all--SugarSync, GoodSync, Windows Live, Mobile Me. They all failed in one area or another--not syncing all files, syncing slow, failing with large files, failing with external drives (I use an SD card on my Sony laptop), not being cross platform, and so on. Then I found PowerFolder. This simple app (with an ugly user interface, unfortunately) works flawlessly. When using it, I know that the "Documents" "Picutres" and "Work" folders will always have the latest file, without fail. Powerfolder has both Mac and PC versions.
Multiple iTunes libraries present their own set of problems, and I have yet to find a solid automated solution that keeps all of my iTunes libraries in sync with each other. The next best thing is a program called SuperSync.
While it's not automated and does have kinks, it's pretty good, and has allowed me to keep my five different iTunes libraries in sync with minimal effort. SuperSync has both Mac and PC versions.
(By the way, If someone out there has a better solution than SuperSync for syncing multiple iTunes libraries, cross platform, on-the-fly without user intervention, I'd love to hear about it.)
Syncing Email, Calendars and Contacts
Exchange 2007 and Outlook 2007 are an unbeatable combination. I've tried Gmail, Entourage, Mac Mail, and others, and nothing compares to the efficiency of Outlook and Exchange. I've installed Outlook 2007 on all of my computers, put Exchange in non-cached mode, and I instantly have access to my full calendar, contacts, task list, and email. My iPhone syncs over the air using Exchange Activesync. Add in Google Desktop to Outlook and I can find whatever I need, instantly, regardless of where I am and what computer I'm using.
The setup above has rendered an offsite or external backup solution unnecessary. If my work Mac fails, my iMac at home has everything. If my laptop fails, I have it all at work, and so on. The chances of everything failing at the same time are pretty much null.