Excellent question!! Permit me to recommend
PLoS - great collection of journals, publishes a lot of neat research in biology & genetics. If you follow the link, you'll see a list of their journals on the right hand side of the page - PLoS Medicine, PLoS Genetics, PLoS Pathogens (my personal favorite) and so on.
Also check out
BioMedCentral, another publisher with some free journals. If you're looking for research on a particular topic, there's no better place to start than
PubMed. If you do a search for a particular subject, PubMed is the best place to start; once you do a search, you can filter your results so that only free-access articles show up (it's cheap, I know, but I do that all the time)