Sapindus is a genus of about five to twelve species of shrubs and small trees in the Lychee family, Sapindaceae, native to warm temperate to tropical regions in both the Old World and New World. The genus includes both deciduous and evergreen species. Members of the genus are commonly known as soapberries[3] or soapnuts because the fruit pulp is used to make soap. The generic name is derived from the Latin words saponis, meaning "soap," and indicus, meaning "of India."[4]

The drupes (soapnuts) contain saponins which are a natural surfactant. They have been used for washing for thousands of years by native peoples in Asia as well as Native Americans.[5] Soapnuts are being considered[6] and used[7] for commercial use in cosmetics and detergents as well as many other products.

Soap nuts have historically been used in folk remedies as a mucolytic agent[8], emetic[9], contraceptive[10], and for treatment of excessive salivation[8], epilepsy[8][11], and to treat chlorosis[8]. While they do exhibit anti-inflammatory and anti-microbial properties, the effectiveness of some of these folk-remedy treatments have not been subject to extensive scientific scrutiny. However, modern scientific medical research has investigated the use of soapnuts in treating migraines.[9][11]

Investigation of the contraceptive capability of plant saponins have shown some spermicidal capacity for certain extracts.[10][12] While the Sapindus saponins have not been proven be as effective as more commonly used spermicides it has been shown that they are less irritating than chemical alternatives.[13]

Soapnuts, such as those of Sapindus mukorossi, are among the list of herbs and minerals in Ayurveda. They are a popular ingredient in Ayurvedic shampoos and cleansers.[14] They are used in Ayurvedic medicine as a treatment for eczema, psoriasis, and for removing freckles. Soapnuts have gentle insecticidal properties and are traditionally used for removing lice from the scalp.

Clothes washing
A few shells in a cotton bag or just 2-3 teaspoons of grounded soapnuts can replace your normal washing detergent and fabric softener. A cotton bag containing a few shells will last for up to 3-4 washes. It appears to work out to be quite a bit cheaper too - you can save up to 50% on normal washing detergent. Soapnuts are fragrance free and if you use the shells instead of powder, they can be added to your compost once their saponin levels have been depleted. Soap nuts are used extensively in some countries for washing woolens and delicates.

Note: While good for normal washing, soapnut isn't great on really stubborn stains (red wine, grass, blood) from what I've read. Warm water is also needed for maximum effectiveness in order to help release the saponin; the higher the temperature, the more saponin is released.
In the case of cold water washing, you may need more soapnuts or probably a better way to go is to soak the bag with the nuts in a bit of hot water for a few minutes and then throw the bag and water in with your wash.
Another great aspect about soap nuts is that they are low sudsing, meaning they are well suited to high-efficiency and front loading washing machines.

Washing Dishes
Soapnut powder or shells can be added to a sink of water to provide you with a totally bridgeable and natural dish washing detergent - and it can also be used in dishwashers! A reader mentioned placing 3 half shells in the cutlery basket, which lasts about 3 loads. Using soapnuts works out to be even cheaper than the cheapest dishwashing powder!

Polishing jewelry
Commonly used for polishing jewelry in India and said to be great for silver and gold - add water to soapnut powder to form a paste, apply then buff off.

Garden
A weak solution of soapnut sprayed on plants can help deter pests such as aphids

Washing cars, floors, window etc.
Boil a handful of crushed soapnut shells in 2 cups of water, simmer for 5-10 mins, then strain. You'll be left with a cleaning liquid concentrate that can be used for washing cars, floors and just about any surface!

Soapnut in medicine
It seems that some folks ingest soapnuts to cure a number of ailments and it's also used in Ayurvedic medicine as a treatment for eczema and psoriasis. Be sure to consult an alternative therapy professional before using soapnut to treat any medical condition. I noticed all sorts of weird and wonderful potions and concoctions around the web for treating this, that and the other- it's one thing to use soap nut to wash dishes, quite another to start eating the stuff. Play it safe, don't do it - I hear they are terribly bitter anyway :).

Soapnuts and fish
Just because something is natural, it doesn't mean it's totally harmless. The saponin in soapnuts in large enough quantities can have a toxic to fish. It's been used for centuries as a way to stun fish in ponds that then float to the surface for easy gathering. Saponin does break down quite readily, so it doesn't pose a significant environmental threat, but as a precaution, don't empty buckets of the solution directly into waterways.

Kosher. Non-irradiated. From India.
Botanical Name: Sapindus delavayi/saponaria officinalis

References
wikipedia.org http://en.wikipedia.org/wiki/Sapindus