Personally I back up my desktop at home as a clone -- so if it crashes I can rebuild fairly quickly (in a couple hours) from that clone and only have lost a few days at most.  

which is pretty slick and works well.   Time Machine automatically backs up all of your data and allows you to restore files before the crash occurred.  Please note: When your external drive is full. some odd things happen; consequently, before the external drive is completely full, I recommend wiping everything out on the external drive and restarting the full backup process at that time.

PCs:  For PCs, there is a variety of solutions available:  Acronis, Carbonite, Mozy, etc.  They all priced essentially the same.  I use Acronis TrueImage Home for my workstation and make a full clone every Friday.  So if my PC dies, I can always rebuild  it in about an hour from the previous Friday.  For the other PCs I mentioned in our home, we use an online service that pulls files from specific folders on each computer when they are online and when the files have changed.  The service is automated and costs about 50 cents per Gigabyte.  You can find less expensive alternatives, but I've found that a few cents more per month makes for better customer service when you're trying to get those files back, which is really the goal.