Website: ProjectManagementHacks.com.
Currently using the SumoMe plugin for sign-ups which is working moderately well and has the virtue of being free.
I'm willing to look at paid plugins (prefer one time fee to monthly recurring) if there is good data & case studies showing the value
I personally use Subscriber's Magnet by MaxBlogPress. This is a one-time fee plugin, but it allows multiple points of engagement (e.g. top of page, bottom of page, sidebar, popin, comment box, etc...) to increase overall optin rates for your site.
My site saw a 32% increase in subscribers after deploying this plugin.