In fact, "th" represents two different sounds in English, a voiced and an unvoiced consonant. (Þink of þe difference between "them" and "thick".) But we never really notice þe distinction; þey operate as one phoneme in English. Þe only modern language þat uses þe letter is Icelandic, but in þeir spelling "þ" is always "thick"; þe softer, voiced consonant is spelt ð.
Apparently þe reason þat "þ" was dropped from English is þat when printing began, þe font sets arriving from France and Germany had all þe oþer letters except þis one, so we slipped into þe situation of spelling þe sound "th". (For þe same reason, we dropped þe old letters wynn - ƿ - and yogh - ȝ - þough þe latter survives in þe name of Sir Menȝies Campbell.) Þis process has left English as a language using very few diacritical marks in its version of þe alphabet, which I have always felt is raþer dull.
Þere are a number of oþer languages which also have þe "th" spelling mainly for þe unvoiced version - Welsh and Albanian are þe two I am most familiar wiþ - þough in Castilian Spanish, "c" and "z" are often used for it, and in Turkmen, oddly enough, it is represented by þe Latin letter "s". In a number of languages (including again Castilian Spanish) "d" generally represents þe voiced sound; in Albanian it is spelt "dh" and in Welsh "dd" (and in Fijian, apparently, "c").
I am not committing to write all my future livejournal entries using þis neglected letter. But I am not going to ignore it eiþer!