Undergraduate students can search for REU programs here:

• AMS

• NSF

• MathPrograms.org

• Steve Butler's list

The following sources can help one find conferences, workshops, and research schools (mostly for probability):

• AIM

• AMS

• Bernoulli Society

• Carl Mueller's webpage

• COMS

• IMS

• IMS mailing lists

• Mathmeetings.net

• MPC mailing list

• OWP

• Researchseminars.org

• Women in Probability

For BibTeX entries:

• MR lookup

• DOI lookup

For journal title abbreviations (these two sources are not always in agreement):

• AMS conventions

• UBC database

For subject codes:

• MSC classifications (2020)

For inciting disagreements: 

• Mathematical Conventions Survey