I don't have a traditional landline.  I have Vonage.  Originally, switching to Vonage was something I did to save a few bucks, as the monthly cost of Vonage, which includes unlimited calling anywhere in the US, was still less than just local service from the phone company.  Over the past year, they've added services that make them even more of a value.  Most of these are "beta" features, ala Google, but they're still cool as hell.

Vonage Contacts:  More than a simple online phonebook, this is a full fledged six-way conference call system with a slick AJAX interface.  Very useful for the telecommuter.

Vonage Me:  A "click to call" feature that allows your contacts to initiate a toll-free call with you without dialing direct.  I've added the URL to my email signature.

Vonage Fax:  An outgoing fax service.  I've really, really needed this service for a long time.  I've tried a few pay services on the web, but this one is included with the cost of my Vonage subscription.

Vonage Talk:  A softphone interface for your primary Vonage line, combined with an IM client.  For some reason, I couldn't get this one to work.  It kept erroneously telling me my password was wrong.

I've loved Vonage for as long as I've been with them, and their willingness to explore new features like this makes me love them even more.  It's too bad they've been having financial problems.  They deserve a far better fate than to be sued into oblivion by the traditional telecom industry.