I realize that most of the big name apps are cross-platform (except for Windows Phone ), but in some cases apps that work on one platform are essentially worthless on another platform. Additionally, some folks have a pathological hatred of an OS that they don't use, that they're going to be frothing over at having their apps mixed in with apps from that other OS. So, if you've got a favorite app, feel free to post it here. * Indicates new item added to a post in the past 30 days. System Apps Lookout Offers anti-virus protection, which isn't really necessary (malware exists for smartphones, but is still relatively rare), but the killer features for this app are that it allows you to log into a website and find out where your device is. It you can also make the device "scream" (i.e. it produces a loud siren sound). Handy, if you've misplaced your phone/tablet in the house and don't know where it is. Quick Boot If you've got a rooted Android device, Quick Boot allows you to easily reboot the device, put it in recovery mode, take you to the bootloader (so you can run a different version of Android) or power your device off. Titanium Backup Lets you back up or delete apps on your rooted device. This is handy if your phone is overloaded with crapware from the manufacturer or phone company. You can backup an app you don't want, then delete it, that way, if your phone acts wonky when you reboot it, you can restore the app, none the worse for wear. It also allows you to restore apps if you've had to wipe your phone and start all over again. The Play Store won't always restore your apps in the case of a full wipe, so you have to search for them, redownload them, and then reconfigure them. If you've got Titanium Back Up installed, you can just restore them from the phone, with all your settings intact. Google Voice Let's you send and receive text messages without dealing with your cellphone carriers limits (if they have any), doesn't accept MMS messages from anyone other than T-Mobile customers right now. You can set it up to be the default phone number on your phone. Phone (AKA GV Phone) hooks into your Google Phone app and allows you to make phone calls using your data or wifi connection. This saves your minutes on your data plan, and it also allows you ditch a cellphone plan completely and use a portable wifi hotspot to keep you connected. This saves you huge amounts of money. Data plans for hotspots typically run $20 -$30 for 2 gigs. Compare that to paying $70+ for a standard smartphone plan. Its not perfect, because you can't dial 911 using it, you have to carry an extra device, and you're not always alerted promptly when you've got a new text, but it beats shelling out big bucks every month, IMHO. Web Browsers Opera for Android The big selling point for Opera for Android is that it reprocesses and compresses web sites before downloading the pages to your device, meaning that you use less data. Important if you're on a mobile data plan with a low cap. Entertainment Related Apps Roku Remote Got a Roku box? (If not, why not? They're awesome!) Then get the remote app, its free and works way better than the remote that comes with your Roku. GetGlue a check in app for movies and TV. The nice thing about it is that you can follow people with similar interests and discover TV shows and movies you might not otherwise know about. Navigation Waze, hands down, the best navigation app out there. Its so good that Google bought them. Real time traffic information, it can route you around traffic jams, shows you the location of speed traps, and lets you know important things in your area (like construction zones). Trapster shows the location of speed traps, and has one advantage over Waze in that not only does it display your current speed, but it shows the posted speed of the road your own. This is handy if you're driving in an unfamiliar area where the speed limits aren't clearly marked. Productivity Evernote A very basic wordprocessor, but has the advantage of being able to by synced across multiple machines and devices. Also allows you to do the same with photos and audio recordings.