Adbase is an online custom list service for design professionals. They tout themselves as a “The easiest way to reach new clients”. After using them for a couple of years, I like them—especially because they make e-mail campaigns as painless as possible.
If you want an alternative mailing list service, then how about Agency Access? They recently acquired Steven Langerman Lists (consolidation goes on—world without end.)
Another great website is Brands of the world. A colleague who used to work at Virgen Advertising mentioned this site once and expressed some reluctance to share such a useful website for fear that popularity might cause its demise. Here it is anyway!