I use CsipSimple. It a standard SIP protocol client. Way better quality than Skype, but it all depends on the service provider.
Generally, if you want to be able to call land lines, you need a service plan from one of thousands of SIP service providers. The rates are very competitive and the sound quality can be the same as any regular phone call.
In theory you don't need a service plan to call other SIP based accounts, but that gets a little tricky as you need to have an access to a SIP server. Some providers will let you call the accounts within the same network for free. Others focus only on land line calling.