http://grants.nih.gov/grants/funding/funding_program.htm#RSeries

Although R awards are aimed towards independent investigators, there are several smaller R grants which could be used by junior investigators to initiate a research program.

R01 - NIH Research Project Grant Program is the most commonly used grant program, and is designed to support a specified research project.  R01 awards typically award less than $500,000 (unless advance permission is given) and are awarded for 3-5 years.

R03 - NIH Small Grant Program provides limited funding for a short period of time to support a variety of types of projects, including: pilot or feasibility studies, collection of preliminary data, secondary analysis of existing data, small, self-contained research projects, development of new research technology, etc. Limited to two years of funding, typically up to $100,000 over the duration of the award.

R15 - NIH Academic Research Enhancement Award (AREA) support small research projects in the biomedical and behavioral sciences conducted by students and faculty in health professional schools and other academic components that have not been major recipients of NIH research grant funds.  Eligibility is limited, see http://grants.nih.gov//grants/funding/area.htm for details.

R21 - NIH Exploratory/Developmental Research Grant Award encourages new, exploratory and developmental research projects by providing support for the early stages of project development.  Sometimes used for pilot and feasibility studies.  R21s offer 2 years of funding of up to $275,000.

R41/R42 - Small Business Technology Transfer (STTR) are intended to stimulate scientific and technological innovation through cooperative research/research and development (R/R&D) carried out between small business concerns (SBCs) and research institutions (RIs).