The following automatic battery-charger design is created with a circuit that could qualify as the simplest window comparator ever built around a single transistor . It starts charging when the battery voltage drops beyond a preset value, and it stops when an upper preset voltage is attained. source

The charger included with the Eee is rated at 9.5v, 2.315A. There isn’t a fixed voltage regulator available for this exact voltage.The circuit designed around LM2576-adj which is an SIMPLE SWITCHER 3A Step-Down Voltage Regulatoradjustable regulator IC from National Semiconductor.This circuit is intended for people who have had experience in constructing electronic projects before. The circuit design and build process are provided simply as a reference for other people to use.source

This is a Li-ion/Li Polymer Battery Charger with ISL6292 which is an integrated single-cell Li-ion or Li-polymer battery charger capable of operating with an input voltage as low as 2.4V. This charger is designed to work with various types of AC adapters or a USB port.The ISL6292 operates as a linear charger when the AC adapter is a voltage source. The battery is charged in a CC/CV (constant current/constant voltage) profile. The charge current is programmable with an external resistor up to 2A.source

Here is a Lithium-Ion Linear Battery Charger with Thermistor Interface with the LTC4050-4.X from Linea technology.The LTC 4050-4.1 and LTC4050-4.2 are complete standalone constant-current/constant-voltage linear charge controllers for lithium-ion (Li-Ion) batteries. Charge current is programmable and final float voltage has ±1% accuracy.source

The circuit presented here comes as a low-cost alternative to charge mobile telephones/battery packs with a rating of 7.2 volts, such as Nokia 6110/6150.source