If you’re a PC user, most commonly used programs, such as Firefox, Thunderbird and Skype, are available as Portable Apps from www.portableapps.com.
They are all free, and most are pretty small, around 30-50MB. The trick is, you can save them to your Dropbox folder and they will be completely synced. So whenever you change your settings, install an addon or reply to an email, that will be synced directly on all your machines.
Even better, if you have to use a public computer that doesn’t have your programs installed, you can download it straight from your Dropbox and have all your settings ready to go – just like home!