Buffer is a website, browser extension, and app that allows you to schedule Tweets for future dates and times. It also allows you to easily post the same thing to different social media networks, including Facebook, Pinterest, and LinkedIn.
IFTTT (short for IF This Then That) is a powerful tool that can automatically link up different services in unique ways. Using "recipes" (you can make your own, but many are premade) you can use your Google Calendar to schedule Tweets, or have your Instagram pictures automatically show up on your Twitter page. You can register for a free account, and they have a mobile app, as well. Check out this list of recipes to streamline your social media to get started.
With a Linkis account, you can share links on Twitter in very customized ways. You can track the links you share to see how many people followed the link, and include information with the link to help boost your own social media presence.
Pocket is a website, browser extension, and app that let's you easily save articles you're reading on your computer, laptop, tablet or phone so you can get to them later. If you come across an article in your Twitter feed and want to come back to it later, simply share it to Pocket and it will file it away to look at later (you can also share articles that are kept in your Pocket on Twitter with just a couple of clicks).
TweetDeck is an outstanding online tool that lets you easily manage your Twitter account. TweetDeck divides your browser window into different columns that will track your Twitter account (or multiple Twitter accounts), as well as let you keep tabs on trending topics, hashtags, etc.