eigenfactor.org is a free source to search for influential journals in a specific research field. It provides lists ranked by influence based on citation analysis, similar to but different in detail from ISI’s impact factor (see Impact Factor and Citation Index on Wikipedia).
Here are direct links to some specific areas:
COMPUTER IMAGING,
MEDICAL IMAGING,
COMPUTER SCIENCE,
FLUID MECHANICS,
MOLECULAR AND CELL BIOLOGY,
NEUROSCIENCE.