This is what I offer to me clients...
- Phone Support
- Snail Mail Support
- Online Medium Chat Support (ICQ/YIM/AIM/MSN/IRC)
- Online Site Chat Support (eg: LiveResponse, PHPLive!)
- E-mail Support
- HelpDesk Support
- Request Call-Back Support
- SMS Support (Used to offer, now only to people in France and possibly Europe)
They all pick:
- Online Medium Chat Support (ICQ/YIM/AIM/MSN/IRC)
- E-mail Support
- HelpDesk Support
I think I offer too many options, but it's nice.

Everything, no matter what, is replied to in at least 12 hours except snail mail which takes about three weeks since my company is in France.