You can use Dropbox to sync things outside your Dropbox folder using a little Linux wizardry.
Sync Mac Desktop with Dropbox:
Open terminal and type this and hit enter:
ln -s ~/Desktop ~/Dropbox
Sync Mac Downloads with Dropbox:
ln -s ~/Downloads ~/Dropbox
Done and done.
Pro tip: the first path is the source, the second is the target container you want to choose (not a copy of the source itself on dropbox). Read here for more info on this.
I do the same thing to sync my MAMP folder on my mac through Dropbox:
ln -s /Applications/MAMP/db ~/Dropbox/Apps/MAMP
And to sync my Wiretap Studio library:
ln -s ~/Documents/WireTap\ Studio\ Library.wtpl ~/Dropbox/Apps