Boomerang (plugin for Gmail): Boomerang allows me to schedule emails for future dates (so I can, say, write something and "send" it while I'm on vacation). I can also schedule emails to return to my inbox on a certain date if I haven't received a reply. There's a free version for personal email & free up to 10 emails a months for business accounts.
Instapaper (plugin for Chrome + app for Android): Save things (articles, blog posts, etc) to read for later, and Instapaper will auto-sync them to your phone (if you have the Android app), your Kindle, or a number of other consumption options. Bonus: Instapaper strips the content down to just the words -- no ads, no distractions.
Rescue Time (app for Mac or PC): This app runs in the background and tracks (with your permission) everything you do on your computer, so that you can learn how productive you are. Since I do multiple part-time tasks at work, I use Rescue Time to determine how much of my time goes to each task. Once you've gathered data for a few days, you can start to categorize which activities are "very productive" (e.g. work-specific tools) to "very distracting" (e.g. Facebook). Bonus: If you need to concentrate and get serious, press a button and Rescue Time will block all activity you've previously categorized as "very distracting."
Mental Health & Wellness
Buddhist Meditation Timer (app for Android): I use this frequently on the bus. The app displays an intentional phrase and a picture, and chimes for 3-30 minutes (you set the duration). That's it! It's a nice way to use the bus ride for something calming. Optionally, you can have the app send meditation reminders to your phone.
Time Out (free app for Mac) makes me take breaks from my work computer throughout the day. It's highly customizable -- I currently have it set up to take 15 second breaks every 10 minutes and 15 minute breaks every 90 minutes. It's a good reminder to rest your eyes and take a walk every once in a while.
Libra (app for Android): Helps track weight trends, rather than single weigh-ins. You can input your weigh-ins by hand, or you can take the super-lazy route like me and have your Withings scale wirelessly sync with Libra. It helps me see trends in my weight, rather than focusing on any single weigh-in.
My Days (app for Android): My Days is an ovulation tracker that learns your cycle as you input your start and end dates. I don't know why it took me until my 20s to figure this out -- before using this app, my period almost always caught me off guard. Tracking my ovulation cycle is great because I can prepare myself (and my husband) for inevitable mood swings.