Many clients don't think to make backups. They admit that if it doesn't happen "automatically" then it just won't happen. For them we really encourage online backups, as the software typically will automatically detect updates and additions to the folders they've identified as needing backup. Here we've compiled a listing of such online backup sites. We're not saying which, if any to use, but we are saying "please make backups!".
We've indicated whether they appear to support MS Windows (W), Mac (M), and Linux (L)
BackBlaze http://www.backblaze.com/ w + M
Carbonite http://www.carbonite.com/en/ W + M
CrashPlan http://www.crashplan.com/ W + M + L + Solaris
SpiderOak https://spideroak.com/ W + M + L 32/64
iDrive http://www.idrive.com/ W + M + phone contacts
SugarSync https://www.sugarsync.com/ W + M
DropBox http://www.dropbox.com/ W + M + L + iPad + iPhone + BlackBerry + Android
iBackup http://www.ibackup.com/ W + M + L + iPhone + MS server apps
ElephantDrive http://www.elephantdrive.com/home.aspx W + M (both require Java)
PennyBackup http://pennybackup.com/ W
GoDaddy https://www.godaddy.com/email/online-storage.aspx W + M + iPhone + Web
Norton Online Backup http://us.norton.com/online-backup/ W + M + Browser
Acronis Online Backup http://www.acronis.com/homecomputing/products/online-backup/ W
Mozy http://mozy.com/ W + M
KeepIt.com http://www.keepit.com/ W + M
KeepVault http://www.keepvault.com/ W
JungleDisk https://www.jungledisk.com/ W + M + L
MyOtherDrive http://www.myotherdrive.com/ W + M multiple computers on one account
SOS Online Backup http://www.sosonlinebackup.com/ reseller program, lots of awards, branded. Rich Steen in CA
Comcast.net http://xfinity.comcast.net/constantguard/Products/CGPS/backup/?cid=NET_33_624 W only?
FireBackup.net http://www.netmass.com/ W + LinuxServer + VirtualMachine, etc
SafeCopyBackup http://www.safecopybackup.com/ W + M + Android + iPad + Smartphone
SecureBackup http://securebackup.com/ W
Concentsus http://concentsus.com/ W
Diino https://www.diino.com/ W + M + L 32/64
Tilana http://tilana.com/ W
ADrive http://www.adrive.com/ W + M + L
LiveDrive http://www.livedrive.com/ W + M
F-Secure http://www.f-secure.com/en_US/storage/ W + M
Backupify https://www.backupify.com/ an all-in-one archiving, search and restore service for the most popular online services including Google Apps, Facebook, Twitter and more.
Seagate eVault http://www.i365.com/ W only?
Many of these services were mentioned at BackupReview.info