For undergraduate students searching for REUs:

• AMS

• NSF

• MathPrograms.org

• Steve Butler's list

For graduate students:

• Advice from different sources

For finding conferences, workshops, and research schools (especially in 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 creating graphics:

• Inkspace (good for pdf vector outputs)

• Mathcha  (friendly interface for creating TikZ pictures)

• Graphic (good for iPad use)

For BibTeX entries:

• MR lookup  (sometimes publication titles are not properly capitalized, but this is an excellent resource)

• DOI lookup

• JabRef  (my preferred bibliography management system)

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