The following is a list of apps which I use on my own stores and can vouch for each and every one of them to be easy to use, cost effective, and work like a charm! In no particular order, here they are:
Coupon Pop - Boost your store's conversion rates by offering your customers discounts for joining your mailing list.
Announcement Bar - Puts an announcement at the top of your screen for your customers!
Minifier - Optimise your photos for the internet to increase your site's loading time.
Photo Resize - Make all your photos the same size, no matter what size they start at.
Signifyd - Greatly reduce your chances of losing money to fraud with their no-chargeback guarantee.
GoldMerchant - Increase traffic by building an affiliate network who earn money by referring people to your site.
MailChimp - Automatically import your customers to your MailChimp List.
Tawk.to - A free chat system to help you help your customers.
Printful - Design your own merchandise to sell.
Geolizr - Geo-targeting apps including currency conversions and country-specific popups.
HelpCenter - Make a great FAQs list!
Hull Social Login - Get your customers to create accounts easier by integrating their social media accounts.
Scroll to top - Create a button that will let your customers get back to the top of your page and navigate easier.
Trackr - Create a better shipping tracking page.
Wishlist - Get a better chance at sales by letting customers create a wishlist.
PageSpeed Guru - Increase your site's PageSpeed results!