JournalTOCs is a free service that allowas you to stay up to date about the newest papers published. It indexes the table of contents (TOC) of more than 24,000 journals, from 2162 publishers.
It's very easy to use!
Go to http://www.journaltocs.ac.uk/, search for a specific journal in the search box or browse the list by publisher or subject.
Once you found the journal that you are interested, you can follow it (a free registration is required) and you will receive information about all the new articles published in this journal.
Metadata are not always accurate, but for most of the articles is also possible to read the abstract. The database also gives information about the accessibility, so that you can immediately know if the journal is open, hybrid or if you need a subscription to read the articles.