iTunes will convert between mp3, aac, and wav (and a few others). Note that converting between two lossy formats is not recommended.
Real and WMA are not free or open platforms, so the available tools are not as widely available. Generally, you'll use Quicktime-based tools for audio/video conversion, so Quicktime Pro is a good tool. Real has a free Quicktime component for encoding, but not decoding (thus, no format conversion is possible). A company called flip4mac makes good WMA tools, but none of them are free.