The Language Log blog highlights the research of Genevieve Von Petzinger into the use of symbols that may be ideograms as a form of written proto-language in Upper Paleolithic cave art as described in a linked TED talk that she has given.
In a somewhat related point, somewhere on my blogging "to do" list is the task of figuring out how many ideograms are in common used by readers of American English, ideally accompanied by examples of the use of ideograms to convey a sound associated with the word for which the ideogram stands (in the tradition of military use of words like Victor or Charlie to spell out words in situations where radio transmission quality is poor) in languages that are predominantly ideographic.
The point, of course, would be to illustrate that the line between phonetic writing systems and ideographic writing systems is one of degree, rather than being an all out either/or alternative.
Some of the common ideograms in American English include:
> "greater than"
< "less than"
There are many other ideograms familiar to readers of American English that can't be produced with a single keystroke.
There are thousands of Chinese ideograms in the proto-typical ideogram based writing system (Coptic hieroglyphics being another such language). But, I'd guess that the number of ideograms widely understood by readers of American English.
Some fields, such as mathematics, make particularly heavy use of ideograms which are typically global in reach across the lines of the languages of the people who use them.