Rice is one of the kitniot mentioned earliest, even in the Talmud (apparently Pesachim 35a, 114b). They establish that rice is not chametz and this is codified later (e.g. Mishna Berura 453, 5). The mishna berura even contains the sentence “Even if a person kneads rice flour and the like with steaming hot water and covers it with cloths until it expands like leavened dough, this does not mean that the rice dough has become leavened, but that it has decayed, and it is therefore permitted to be eaten on Pesach.”
Rice seems to be best known kitniot (actually, what is the singular of kitniot?), it is the one example that can be found on virtually all the web pages that contain the word "kitniot". Many people envy sefardim because they can eat rice – actually that is not true, most sefardim do not eat rice (only Tunesian my contacts say, but I haven’t verified that claim). The sefardim who eat rice should examine every grain to be sure it is not chametz – are you sure you still envy them?
- Danger of chametz traces: Apparently a concern seems to be rice enriched with vitamins that contain chametz (Source: OU)
- Danger of confusion with chametz (raw): I would have told you no way somebody confuses raw rice with grains, but after seeing the above picture I stand corrected.
- Danger of confusion with chametz (processed): Rice flour looks like chametz flour. Rice dishes don’t really, maybe some of these unusual varieties might.
- Botanical categorization: Cereal grain
- Known in 13th century: Yes.
- Verdict: Kitniot.
Rice photo (c) IRRI