This page points to backups of important US Census Bureau data that has been backed up by Ian from Census Reporter so that the data may continue being of use after a potential government shutdown in 2018.
A similar effort was undertaken in 2013, but only after the government shutdown. Although most of the links are stale now, Code for America's index of that data is still around.
It is important to note that I only backed up raw data. If you want programmatic access to Census data, you'll need to look elsewhere. If you're looking for ACS data via API, take a look at the Census Reporter API.
https://www2.census.gov/programs-surveys/cps/as of 2018-01-18.
https://www2.census.gov/programs-surveys/acs/tech_docs/as of 2018-01-18.
https://www2.census.gov/programs-surveys/acs/summary_file/2016/as of 2018-01-18.
https://www2.census.gov/programs-surveys/acs/summary_file/2015/as of 2018-01-18.
https://www2.census.gov/geo/tiger/TIGER2016/as of 2018-01-18.
https://www2.census.gov/geo/tiger/TIGER2017/as of 2018-01-18.
https://lehd.ces.census.gov/data/lodes/as of 2018-01-18.
https://www2.census.gov/programs-surveys/popest/as of 2018-01-19.