My favorite site to discover new apps and software is Product Hunt at producthunt.com.

Every day, there are typically a dozen or more posts about new products. The best are voted up, and often the product creators will post special discounts just for members of Product Hunt. It’s free to join.

Product Hunt is also a great place to search when you need a certain type of software, for example something to help you design mobile friendly email layouts which I was looking for today.