The Swazi currency is the Emalangeni. It is pegged to the South African Rand and the Rand (notes, not coins) is accepted universally.

Try and change any excess Emalangeni back into Rand or another hard currency before leaving the country  as it will be hard to do so outside Swaziland.

There are VISA ATMs in major towns in Swaziland, dispensing Emalangeni.