CD/RW is the best for the home users theses days and probably the most economical as well. Tape drives are mainly used by servers and corporate companies who have a large amount of data. The external HDD are good as well, but the downside to them is that if you aren't able to boot into you OS, accidentially format the wrong drive, hit by a very distructive virus then you are hosed.
<b>External HDD</b> - cheapest (if you already have the desigante free space)
<b>CD/RW</b> - most economical
<b>Tape Drive</b> - Expensive (best for large amount of data)
<font color=purple><b>An average human brain weights aprox. 3lbs., but a thought weights a load.</font color=purple></b>