The thorn (Þ) and eth (ð) are very old letters that used to be used in English and are still used in Icelandic. Both represent a "th" sound. We have the thorn character to thank for the whole "Ye Olde" thing in English. In Middle English, "the" was written as a thorn with an "e" superscript. The thorn shape gradually morphed into a "Y" and the Y-superscript-e combo continued to be used for "the" well into Modern English. So "ye" is actually pronounced "the." (This digression brought to you by Brad's History of The English Language professor).
