I am currently using Constant Contact (constantcontact.com). Top Producer is another great choice. I have been a real estate technology trainer for over 10 years, and Top Producer has always been one of the best choices for contact management. ACT is great, but it seems to bog down the computer, and it's limited to the one computer you are using unless you have the network version or the online version. Some other great programs to check out are Sharper Agent (sharperagent.com) and iContact (icontact.com).
Choosing one really depends on the features you need. What makes Top Producer so great is the automated followup system (action plans). With some setup, you can automatically schedule calls, print letters, and auto send emails to clients consistently. I chose constant contact because I don't do mailers and really only need a strong email program.
I would be happy to answer any specific questions you may have about any program you are considering.