I hold an Indian passport and currently studying in the US on F-1 visa. I'll be attending an academic conference in Canada and I have been accepted as a student volunteer. The conference is for 5 days and I have an obligation to volunteer for 20 hours at this conference. I won't be paid for my volunteer work but my registration fee will be waived. Do I need a visitor visa or a work permit in this case? The online tool on Canada's immigration website is confusing me.
Presuming you mean "find out if you need a visa" I didn't find it confusing. You are a citizen of India without a US green card, so you need a visa. Your purpose in coming to Canada is to attend a conference or trade show. The website says you need an ETA for a business visit and can apply online. It didn't ask me if you were going to do any volunteer work.
If the purpose of coming to Canada was to do volunteer work (eg on a farm for the summer) then perhaps the tool would have led somewhere different. But your purpose in coming is to attend the conference. You still need to meet the other eligibility requirements, but since you are a student in the US you need to leave Canada to complete your studies, so that shouldn't be a problem.
Still feel worried? Try the definition of work on the government website, which includes:
... other types of unpaid short-term work where the work is really incidental to the main reason that a person is visiting Canada and is not a competitive activity, even though non-monetary valuable consideration is received. For instance, if a tourist wishes to stay on a family farm and work part time just for room and board for a short period (i.e., one to four weeks), this person would not be considered a worker. Work on a farm that is expected to extend beyond four weeks would require a work permit.
By that logic, less than a week at a conference where all you get is conference admission would not be work.
Disclaimer: I am not a lawyer or an immigration consultant. I am pasting stuff into a box from a government website. Make your own decisions.