It used to be common to split the Data and Log parts of your database across different disk drives. Then people started buying servers with single large RAID arrays and kind of forgot about this. However, for not too much money you can buy a RAID controller with two channels and extra disks for multiple RAID arrays. At it's simplest you can put the data file on one IO channel and the log on another to spread the load. If you want to get more clever, you can allocate some tables to one database file on one IO channel and the rest to a file on another channel and spread the load this was.
Some people choose to put the transaction log file on a non-RAID device.
This is a perfectly reasonable thing to so provided you are happy with not having any fault tolerance for your transaction log. There are not one , not two , but three Technet articles worth reading on the subject of using cacheing disk controllers with SQL Server. There are a couple of SQL Server configuration settings that are relevant to disk performance - take a look at the Software pages.