To name a few:
PLoS Journals
eLife
Science Advances (Science Magazine's new open access journal)
Genes & Development
Journal of Biological Chemistry
If you want to read up on a specific topic, go to PubMed (
http://www.ncbi.nlm.nih.gov/pubmed) and select the "Free full text" option under "Text Availability" on the left hand side. Then put in whatever search criteria you want.
As far as MCAT prep goes, I would recommend you stick to one basic science topic you found interesting in your classes and do the following:
(1) Read a review(s)
(2) Practice reading and analysis on simple, short research articles. You may find some at journals that specifically publish short reports, such as Nature Communications.
(3) Move up to full length papers with more complex intricacies.
Depending on your academic background, you may find it hard to jump straight into primary literature because you might not even know which experimental conditions are actually controls.