Dropbox is a file sharing application that puts a folder in your PC that is sync with any dispositive you have the dropbox application on. You can then give access to anyone with an email to these files to be able to read or even edit them.
Google Drive another file sharing application, but with the whole Google Integration.
Trello its a project management web service that organizes your tasks into boards, lists, and cards to enable your company to prioritize your projects in flexible and rewarding way. Uses a Kanban style board, but can be adapted to any methodology.
Gmail for Business gest a custom email like [email protected] and utilize the full power of a Gmail account. Build customer trust by using your domain name, and have the email management from google with the best uptime and features for emails.
Slack is a business messaging platform. It has options to make public and private channels for teams, customers and anyone you want. You can share files, track time, and have automation on the way you message your team. Has video calls and team video calls. It can be a great element inside your business to organize the flow of information. It can be used on PC, apps and web.
Whatsapp allows text messages, voice and video calls. You can also send images and other media, documents, and your gps location. Has a great adoption by all kind of people, and has a business option so you can automate some of your interactions with your clients. Requires a Mobile phone, but some functionality can be used in the web version.
Skype an old school messaging app. Has text messaging, video calls and the usual stuff.