Bloomberg Law, Lexis Advance, and Westlaw all provide the ability to set up alerts for legal topics and have those alerts sent to you via email. You can choose how often the alerts are sent.
Don't forget the power of KeyCite and Shepard's. Use these tools to monitor case developments and pending legislation.
Many current awareness tools give you the opportunity to sign up for newsletters, to be delivered daily, weekly or monthly or as breaking news happens. Broad coverage is available through the National Law Journal and Law360, both available from Lexis, while other tools are subject-specific, like E&E Daily and the Environmental Law Reporter for environmental issues, and LawMemo for employment issues. Bloomberg Law has a number of law reports; these vary in topic from class actions and tax to labor and employment and health care.