Below is a list of parishes throughout the Archdiocese of Baltimore that broadcast Masses live online.
These are great resources for those who are unable to make it to mass.
EWTN on cable and satellite TV also broadcasts mass.
Radio Mass of Baltimore
The Radio Mass of Baltimore broadcasts live from St. Ignatius in Baltimore every Sunday at 9:03 a.m., with an encore broadcast at 6:03 p.m., on WCBM-680 AM.
Cathedral of Mary Our Queen, Homeland
Monday through Saturday at 8:15 a.m. [except July 1 – August 10, when it will be the 7 a.m. Mass – except the 8:15 a.m. Mass on July 4], 5 p.m. Saturday, and 8 a.m., 9:30 a.m., and 11 a.m. Sunday.
Church of the Nativity, Timonium
Sundays at 9 a.m., 10:45 a.m., and 12:30 p.m. Mass rebroadcast at 2 p.m., 4 p.m., 5:30 p.m. (with chat),  7 p.m. and 9 p.m. on Sunday afternoons.
St. Elizabeth Ann Seton, Crofton
Daily, Monday – Thursday, 9 a.m.; Sundays at 9:30 a.m. (Archive of daily and Sunday Masses on YouTube)
St. John the Evangelist Catholic Church, Wilde Lake Interfaith Center, Columbia
10:30 a.m. and 5:30 p.m. Sundays; On your computer: visit the link above. 
Click above for links to other Catholic programs, homilies, podcasts and more
Other resources such as the Word Among Us, the Magnificat provide daily mass readings and meditations.