Now owned by Cisco, the WebEx MeetMeNow Web conferencing tool includes audio and video conferencing in its flat fee charge. It also offers application/desktop sharing, online chat (both individual and group), file transfer, moderator functions. Offers a reseller program. Very good email support.
One caveat is that this particular service can only accommodate up to 10 attendees per meeting. If your client thinks there may be times more than 10 people will be involved in a meeting, it's best to go with an application that can scale up, including Cisco's other service, WebEx.
Supported platforms and browsers: Windows 98, Windows Vista, NT 4.0 SP6, 2000, XP (Home and Pro), 2003, IE 6 or later, Netscape 4.7 or later, Firefox 1.0 or later, and Mozilla 1.6 or later.
Fees: Monthly: $39 with prepaid annual plan for up to 10 attendees; $49 includes unlimited meetings for 10 attendees.