I am using DropBox for 2 months. I am using it to sync files between different machines (my PC at work and my 2 macs at home) and for file sharing (i have used it to upload my 2 mp3 mixes, see older posts).

Yesterday, I discovered another killer app called ioj that seems to be a very good solution for file sharing with friends and colleagues. Watch ioj in action video at the website to get a taste. Mac version is out around Feb 20th.