Since I have decided to abandon Windows and Mac OSX permanently for Ubuntu Linux many of the programs I feature will be exclusively for Linux. Fear not though I have been using all operating systems for so long now that there should be something for everybody.
A good place to start is the very attractive Linux email program Geary. Linux has the reputation being of purely functional without consideration of aesthetics. I will say that in the early days there may have been some truth to the preconception but now Linux programs like Geary rival Mac OSX when it comes to beauty and simplicity.
To install Geary on Ubuntu Linux open the terminal and paste these commands:
sudo add-apt-repository ppa:yorba/ppa
Press enter to add the repository
sudo apt-get update; sudo apt-get install geary
Press Y and enter again to install geary.
For other non-Debian distributions you will probably have to compile the source code.
Oftentimes I need to test a new Linux distribution or I need a single Windows or Mac Program. Instead of wiping my computer or eating up space with a triple boot computer, I use Virtualbox. Virtualbox allows you to install an operating system on a fake or virtual computer that uses your computers resources.
Carbon Copy Cloner
One of the easiest backup solutions for Mac OSX is Carbon Copy Cloner (CCC). If you have a external hard drive handy and are willing to pay 40$ for the software you will not be disappointed. CCC keeps an exact copy of your mac on an external hard drive. It can be setup to automatically backup your computer on a schedule and keep copies of files that are edited or deleted over time. The major advantage of CCC over the built in time machine is that the backup is bootable. Bootable meaning that if your hard drive fails or you want replace it you can still startup your mac from your backup disk and eventually copy your backup to a new internal hard drive.
If you find that your constantly roving between many windows machines some of which are for public use you may wish that you could bring some of your programs or files with you. Portable Apps does just that by creating a modular applications menu that you can install on any flash drive. All the applications are designed to use your flash drive to store data so bookmarks, emails, etc. are always available. I love portable apps but I wish it had cross operating system support for Mac and Linux.
Windows users might be a little envious of he fact that you can make a Linux desktop look however you want with support for themes, custom icons, and all manner of interactive widgets. A great tool to achieve similar effects on Windows is Rainmeter. There is a bit of a learning curve but it is well worth the effort if you have some RAM to spare. Check out the Reddit or DeviantArt pages for some great examples.
Syncthing is a free and open source method to sync files between computers (Windows, Mac, Linux) and mobile devices (Android & iOS). Unlike popular services such as Google Drive and Dropbox there is no remote server that stores all your files and passes them to each device. Syncthing uses a concept akin to encrypted BitTorrent to transfer files between computers. The advantage to using Syncthing is storage is limited by hard drive size and your private data is not floating around on some private server that may have dubious privacy policies or may be a target for hackers. Syncthing is best if it is running 24/7 on a low powered computer like a raspberry pi or odroid with a hard drive attached. That way files are copied to at least one other source almost immediately and can be transferred to other devices when they are turned on and connected to wifi. You can install a binary Syncthing that is controlled from a web browser but there is also an application called Syncthing-GTK for those looking for an easier to use option and instant file synchronization.