mamierock and tawnyleona seem to swear by k7.net, but their 30-day inactivity deactivation warning rubs me the wrong way. I guess that's what you get for a free service.
At the moment, I'm looking at jConnect and SkypeIn. Does anyone have any experience with either of those, and does anyone have any other suggestions as far as places to check out? I'd also be interested to see if anyone's played with Skype at all, since SkypeIn seems to be attached to it. From what I can see so far, I'm slightly favoring Skype, both because of price and ease of access.
[Update @ 9:25]
Trying out Skype and, at least for outgoing calls, it works quite well. Still debating their incoming call service... as cheap as it is, though, it should be a no-brainer.