Google Calendar or a similar app, since I have to organize my day with every task I have to fulfill, otherwise I forget and it`s not good for my professional and private life
Google Hangouts, as well as Viber, since I have to stay connected permanently with customers and secondly, to socialize with my friends, since they tend to see me quite rarely, due to my job
Google Drive, it gets me rid of the storing headache. I have everyhting in one place: documents, multimedia contents and the possibility of backing up, just in case