LDO Regulators

The dropout voltage is the minimum $V_{in} - V_{out}$. A 7805 needs about 2 V; an LDO (e.g. MCP1700, AP2112, LM1117 at ~1.1 V) needs as little as 0.1–0.4 V — ideal for producing 3.3 V from a single lithium cell (3.0–4.2 V).

Pay attention to quiescent current in battery products (MCP1700: ~1.6 µA) and to the output capacitor requirements — some LDOs need a specific ESR range to remain stable.