We use "ProvideSupport.com".
All of them are extremely easy to use. Basically, you chat out of a popup window on your browser...when someone asks a question, it rings. We pay $15 a month.
My experience is it's still a foreign feature to most visitors. They don't understand there's actually a live person answering their message.
We find we get about a ratio of about 6-1 of phone calls to text chats. During our chat hours, we probably get about 1 out of every 200 visitors using text chat. More encouraging though, probably about 1 in 10 of those people goes on to buy something.