Good filter
Heater or Chiller (depending on what temp you'd like to keep the tank at)
Substrate (I use play sand from the big chain hardware stores. You can also use crushed coral or something similar)
Protien skimmer (very important IMHO)
Lighting (if you're going to keep certain inverts and or corals)
Powerhead (only needed if you require more water movement that your filter can provide)
R/O Unit (unless you have great water from the tap in your home. You can also purchase water at some pet shops or from a local store)
Test Kits
PATIENCE!!!
If you're going to try your hand at saltwater I'd say patience is the most important requirement. That and a commitment to work at upkeeping the tank.
Saltwater is just as rewarding as freshwater and not as scary as most people think. It is a commitment however. Just like with freshwater. IMHO though it's a little more difficult because more factors come into play. There's unwanted hitchhikers on live rock, nuisance algea, etc... Before starting I'd find a good book on maintianing saltwater aquariums. Reading everything you possibly can before jumping in is the best way to prevent failure and frustration.