Myself, I'm using . It's a bit pricey but it has been well worth it to me so far. As Alice mentioned, is also excellent and a bit less expensive than ARPlus. I know many people using it and like it. These two are script based and they are installed on your site.
For a third-party autoresponder service, I'd look no further than . I think they are the VERY best AR service out there. They've been around the longest and have proven themselves. If I'm not mistaken, I believe Lynn here at SSWT is also using them.
BUT ... Be aware that when importing your own leads into these AR services, most require that they go thru the "verified opt-in" confirmation process again. With , they might work with people on a case-by-case basis regarding previously "double opt-in" leads.
Hope this helps some ...
Rick Wilson aka CorpRebel