NRG-Loops are source-level abstractions that allow an application to dynamically manage its power and energy through adjustments to functionality, performance, and accuracy. The adjustments, which come in the form of truncated, adapted, or perforated loops, are conditionally enabled as runtime power and energy constraints dictate. NRG-Loops are portable across different hardware platforms and operating systems and are complementary to existing system-level efficiency techniques, such as DVFS and idle states. Using a prototype C library supported by commodity hardware energy meters (and with no modifications to the compiler or operating system), this paper demonstrates four NRG-Loop applications that in 2-6 lines of source code changes can save up to 55% power and 90% energy, resulting in up to 12X better energy efficiency than system-level techniques.