There are a couple of nightclubs on Koh Samui but by far the best is The Green Mango. I was in Thailand last summer for a month and spent five days on Koh Samui and we ended up at The Green Mango on three of those nights. It's slightly off the street but is well worth a visit. Its really big with pool tables on one side and the dance floor on the other side with a bar in the middle. When we went it was full of tourists like us so it was a great place to meet other travelers. The music was varied and although we didn't know many of the songs played the odd classic crept in. The drinks are really cheap it was free entry for us to get in because we were girls which was fantastic! I would definitely recommend going to The Green Mango if your on Koh Samui as it is a really good night out.