Our company is an on-line technology group featuring web chat, email marketing, customer analytics, and automated proactive marketing; www.intersighttechnologies.com
Adding chat to your site can be very effective in learning your true customer and their needs. Our product is a little more expensive then the ones mentioned here so far as it includes analytics of the chat and provides an private SSL database for each company. Having a way to capture the input from your customer, and reviewing it is a simple analytic report, provides great insight for meeting your customers expectations on the web.
We also believe that the pop up window chats take away from the experience of the customer as they cover your web site during the conversation and require a download of technology to the users PC. The pop up chats have to overwrite any blockers or filters your customers may have chosen to use. This happens automaticlly so the customer doesn`t know or have to do anything. The idea is the customer ask for a chat so has given permission to over ride their personal settings.
We use an in browers method so the web site is never covered and no download is require for the chat to work. A little more sophisticated approach that has some cost, but we think a better way to service a customer. Our chat works through normal Internet protolcal and the customer can move around the site as they please while the chat is going.
These tools are highly effective but you do need to think through there implementation and how you will staff, react, and use the tool to increase the positive experince on your site. Should be part fo your customer strategies and overall marketing plans.
Sorry for the lenght of this - it is a topic I like to write about.