More casual than penultimate is second last, second to last, or second-to-last. I did some queries in the COCA and BNC corpora, and it seems that second last is the more common form in British English. Second to last / second-to-last are more common in American English. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.