Depends on what you want to sync.

I use Dropbox for sync between laptop (ubuntu) and desktop (windows 7), but there is no client for N900. I download what I need from the web interface--not ideal, but it works. I am not sure I would want to have that syncing all the time on my N900. There is 2 gig of space for free, and I don't need all that communication and power drain on my phone. For example, whenever I use a file that gets altered (like Keeppass creating a lock file) on one computer, the other computer syncs it up and shows me a notice.

I use tomboy/conboy on all three platforms. It is like a combination of yellow sticky notes and a wiki. Fantastic program, if you are not familiar with it. And Ubuntu (on Ubuntu one) gives me the ability to sync it among all three platforms. Only drawback is that sync is manual. But this is my workhorse for info sharing. I can keep a list of servers and ip addresses, the size of the air filters in my A/C unit, the light bulb wattages I need to buy, random notes on a project, etc., and fairly easily (though not automatically) have it available everywhere. Ubuntu One even has a web interface to get to the notes if you need it.

Firefox Weave syncs up my bookmarks and other Firefox stuff, which is nice, but I don't seem to use firefox all that much on my n900.

My email, calendar, and contacts sync up everywhere because I use an exchange server. Since I started using exchange with my phone it has become a must-have feature for me. It is what kept me from even considering the iPhone when it first came out, and what keeps droid out now. I MIGHT have violated that rule for the N900, but it is hard to say. In any case, my info is kept synced up very well through exchange. (Just to drag this point out further, I have considered Zimbra and Google to replace exchange here, but nothing so far has pushed me over the hump off of exchange.)

Opera (on the desktop) has some pretty nice sharing capabilities that might also be useful to you, though it is a bit different than "cloud" offerings and I have not come up with much real usage for it.

In theory it would be nice to have a continually backed up directory on my n900 that syncs with my laptop and desktop, but in reality I don't have many apps on the N900 that use files generated on the desktop/laptop and vice-versa. I have 90% of my info-sharing needs covered in conboy/tomboy. So I am interested to hear what your use cases for this are (i.e. what types of files you want to keep in both places and what you do with them?).