Skype are doing a deal right now on their monthly subscriptions. There is a 25% discount on the prices shown, when you sign up to a 12 month subscription. . For example, unlimited landline calls within the UK would cost you about £2.54 per month (including VAT) if you subscribe to Skype. If you want European or worldwide, it's a bit extra.
I use Skype all the time, but I only use the Skype credit I have added to my account, so I am considering one of these subscriptions.
Now of course, before you jump in and buy this, be sure to price compare with other providers. For example, I saw some people mentioning in another thread that they preferred SkyTalk, but I'm not sure of the exact terms and conditions of it.