I'm not using Constant Contact, but I did switch to Aweber.com I was sending several hundred emails via bellsouth and ACT to a local club. After a certain point, they would not let me send that number anymore and I was having to split the list. So I had to do something.

Aweber flat rates their prices. So for $179 year, I can do pretty much unlimited emails up to 10,000. No per email/compaing charge. The price is good for the first 10,000 and $9.95 for the next 10K and so on.

But not to have to manage the list with the sub-scribe/un-subscribe is worth the price alone. They probably have better delivery rates than I could ever get.

I put a form on the website and they have to opt in, and it's all on auto-pilot. Anyway there are several companies that will do this, but Aweber has worked the best for the money for me.
I've probably been using it two years now. I just recently tried some trial softwares to do my own lists for another business, but could not get any of them to send through by ISP. So if you go that route, make sure you can test it first or get a money back guarantee. Of course even if it works today, your ISP could change their rules tomorrow. I also wanted to be able to get a copy of my list, just in case someone goes out of business. You never know these days, but with this service I can at least download an excel spreadsheet and maintain a seperate list.