By this I don't mean web sites where you can type in a word or phrase and get a translation.
I mean PDF copies of old dictionaries that have fallen into the public domain.
I have found some interesting items recently.
Quite a while back, I found out that the Internet Archive had the "Syllabic Dictionary of the Chinese Language Arranged According to the Wu-Fang Yuen Yin" of S. Wells Williams. This is interesting because it's a fairly large dictionary (it has some mistakes, but there's also a book online at the Archive, by Giles of Wade-Giles fame, listing many of them) but also because it has extensive coverage of the different pronunciations of the characters in various dialects.
More recently, however, I came across the Chinese and English Dictionary of W. H. Medhurst. Both volumes of this two-volume work are on Google Books. It claims to contain all the words in the famed K'ang-Hsi dictionary - which covered 47,035 different characters. However, most of the characters in that number are simply in a list at the back of the volumes of simplified, mistaken, or obsolete characters giving the correct characters of which they are duplicates. This still makes it a very useful reference, as no other Chinese-English dictionary comes close. However, this one is very old; so old that the system it uses to give the pronunciation of the characters does not distinguish between pairs of letters like b and p (or p and p', depending on the system you use).
Basically, advanced students of Chinese requiring a really comprehensive dictionary have had to also learn Japanese, at least to a limited extent, to use the one comprehensive dictionary in existence - the Dai Kan-Wa Jiten of Morohashi. This has recently changed, though. Mainland China has come out with the Hanyu Da Cidian and the Hanyu Da Zidian recently, and Dankook University in Korea has come out with a comprehensive Chinese-Korean dictionary.
But I've found that a Chinese-Chinese dictionary from 1915 is also available (in a 1927 printing) on the Internet Archive. The title is recorded as "Zhonghua da zi dian", which isn't quite right, Zhonghua Da Zidian would be better. This is a four-volume dictionary which started from the K'ang-Hsi dictionary, but which was modernized and expanded. All four volumes are there.
