I've been using Campaign Monitor for quite some time now and I love it - it's so easy to use for both myself and clients, it does exactly what I need it to do and the help and support is second to none.
I've been trying to find an online system that could deal with our customer support (they email in/fill in a form and it gets logged in a system where you can reply, assign it to others, track the progress etc) but I can't find anything that matches up to the ease of use of CM. Does anyone else here use a system like this that they could recommend?
One CM clients -- Sifter, http://www.sifterapp.com/ -- offers a product that might help: see also http://www.campaignmonitor.com/casestudies/sifter/
We use a very cool app called HelpSpot for all our email support. It's pretty awesome and I wouldn't hesitate to to recommend it.
You can also try Zendesk. Quite impressive as well.