Yep Chillyd is right. Aweber is one of the best solutions for opt-ins and building your list white hat style. When compared with other solutions their starting price of $19.95 a month is a bargin. They could be charging alot more like their competition, but I'm glad they are not.
The user interface could be a little more helpful when starting out, but they have guides to walk you through steps that you are unsure of.
Also just to point out..if you already have an email list, you can't just import those emails into the aweber database. You will have to re-opt in those members i.e send your people an email asking them to opt-in to your new email list.