You could also try
www.mailchimp.com I use Constant Contact for one of my clients, but I use mailchimp for my business. It's not as user friendly as Constant Contact, but it allows you more flexibility with your design. You have to know a little bit about html emails to begin with, but you can pretty much create any design you want. They also have a 30 day free trial that you can send out to 100 contacts.
As I mentioned in another post, the nice thing about this service is that you can sign up for a month to month membership. If you have no emails to send in a certain month, you don't have to use it or pay for it that month. Much better for small businesses so you're not stuck in a contract.
The other thing I really like is that you don't have to have mailchimp listed anywhere on the email. That way you could offer email newsletters to your clients and it doesn't look like it's coming from some service, it looks like a completely original design. You can have multiple lists set up, so you can have one for your business and then separate lists for each of your clients.
Another service you can offer your clients - and an additional revenue stream! Plus, there's very little you have to do to manage the lists. If someone wants to unsubscribe and uses the link in the emails, mailchimp takes it off automatically. Believe me when I say that's a huge timesaver. I've done html email newsletter blasts on my own and had to manage a 1000 person database. It takes alot of time to search through all your contacts to take someone off the list, or to check to make sure someone has already unsubscribed before you put them back on the list.