While most people are using broadband internet connections, or stealing WiFi illegally from there neighbors there is still people that are using dial-up internet. Either for there main connection to the internet, or as a back up, or for traveling. Years ago there was plenty of free dial-up service providers to choose from, starting with NetZero. It was mostly ad supported, and very limited. People figured out how to disable the ads, then it was patched, then they figured out how to disable them again. This was a constant cat and mouse game with NetZero and other similar services and the crackers, until finally most of the free dial-up ISPs just ended up going with a subscription model.
The good news is that there still exists several free dial-up ISPs. While I have not tested any of these, since the first thing I remove from every PC that I own is the 56K modem, and my portable PCs are netbooks with no dial-up capabilities, you should hopefully find something worth the cost of admission (free) for your dial-up needs, whether it is for your main internet connection, a back up when your DSL or cable modem goes down, or while traveling.
claims to have local access numbers across the country
has access numbers for CA, DE, NJ, PA
has access numbers in the entire state of MI according to the site
has access numbers in NH, Mass, and RI
it appears I may have spoken too soon in the above post, netzero claims to have nationwide access numbers, but only gives you 10 hours free per month.
NYC's free ISP
ID and WA access numbers
southern california access numbers
If you have any others, or want to share your experience about one of the above, then leave a comment below.