RealPlayer, free download here.
120 MHz processor computer (Windows) or PowerPC 604 processor 200 MHz or faster (Mac OS)
16 MB RAM (Windows) or 32 MB RAM and 64 MB memory (Mac OS)
16-bit sound card and speakers
Windows 95, Windows 98, Windows 2000, Windows 2000 ME (final release version only), Windows NT 4.0 with Service Pack 4 or Mac OS 8.1 or later.
28.8Kbps modem
Internet connection and web browser (Internet Explorer 4.01 or later or Netscape 4.05 or later)

For optimum performance

200 MHz processor computer or faster or PowerPC G3 processor 233 MHz or faster
Windows 95, Windows 98, Windows ME, Windows NT, Windows 2000 or Mac OS 8.5 or later
32 MB RAM (Windows) or 64 MB RAM and 128 MB memory (Mac OS)
Full Duplex Sound card and speakers
56Kbps or faster modem
Internet connection and web browser (Internet Explorer 4.01 or later or Netscape 4.05 or later)