Recently I moved to using vonage [1] for my landline and I have to say I’m very impressed.
 
After an initial problem with the courier I received my preconfigured linksys RT31P2 [2], which is a broadband router with 2 phone ports. All I had to do was plug the RT31P2 into my existing wireless router and plug the phone into the RT31P2. I was strangely disappointed that I didn't have to configure my new toy but after logging into the built-in web based admin panel I found that i could configure all sorts but had no need to. You configure the phone settings via the vonage website which of course is obvious as it and IP base phone system but kind of makes you wonder what could happen if someone one other than myself got access to it - something like change my call diversion to a premium number set the wait time to zero seconds and then call me. I've emailed the support about this suggesting that they send and email, txt or an automated call when ever you update your call diversion so that you know.
 
The call sound quality is very good; I had originally thought it would be a noticeable decline in quality but no. So I get free calls within the UK and down south and there are some great extras that all come under the £7.99 p/m line rental. I especially like the ability to have me voicemail emailed to me and the ability to see my call costs updated after each call rather than waiting for the bill every month/quarter. The only downside was that I couldn’t transfer my existing landline number. Also in the event of either my broadband connection failing or a vonage outage all my calls get rerouted to my mobile, hopefully I'll never have to use this "feature".

I’d kept my original line just encase I found vonage’s service to be really bad but I’ve now decided to move permanently :)
 
[1] http://vonage.co.uk/
[2] linksys RT31P2: broadband router with 2 phone ports