Cloud technology is spreading wide and there are various applications environments, i think there are both pro (keep data safe and everywhere and everytime available) and cons (uploads/downloads times).
A list of actual uses are:
store and share online data
photos, docs, audio, videos, archives etc…
famous examples are Google Docs, Dropbox, Amazon S3 services
hdd backup images
many disaster recovery products, both free and not, offers the feature of online backup (eg. Comodo Backup)
eg. antiviruses and malware removal tools like Panda Cloud, Comodo Cloud Scanner, IOBit Cloud
web apps and CMS hosting
eg. using Amazon EC2 services the applications are deployed online
play via cloud for example with OnLive service
And do you use them? In which environments?