Box.net is out of the question as it excludes larger files.
Mozy is more of a backup solution while Dropbox is a sync, collaboration/sharing tool (on top of being a backup solution). If you will just backup your drive I would recommend Mozy as you will get more space for your money. If you would like to do what Dropbox does best its really no question about it.
Personally I'm fine with Dropbox free version. I use it to have my most important files with me in my iPhone all the time (via Dropbox iPhone app). For backups I simply use Time Capsule as well as a monthly off site backup (a hard drive placed elsewhere).
If you register for Dropbox make sure to do it from a referral link such as this one:
https://www.dropbox.com/referrals/NTE3NTY3Mzc5
It will give you 250 MB of additional bonus space from start (2.25 GB). By time as you invite others you can get up to 8 GB (!) of additional free space (exceeding 10 GB). Now that's quite good.