There are plenty of telephony apps out there but perhaps few as simple as Telephone. However, Telephone is not designed for Skype users - it's for those who have their own server configured.
Telephone is extremely lightweight and simple. It can be used to call regular phones via any Session Initiation Protocol (SIP) provider so if your office or home phone works via SIP, you can use that phone number on your Mac anywhere where you're connected to the net.
When you setup Telephone, you have to enter your name, SIP address, Registry server and e-mail address. The SIP address and Registry Server you'll either have to obtain from your ISP or telephony provider and this could prove the hardest part in setting-up Telephone. However, if you do get it up and running, you'll find Telephone integrates perfectly with existing Address Book contacts and allows you to use your land line number wherever you have an internet connection.
If you're looking for SIP software and have been disappointed either by prices or service, then you'll probably find Telephone the perfect solution.