International SMS : free

These are my short-cuts for accounts that are set-up and ready to use:

twitter : UK, Belgium
voipcheap : Italy

voipcheap allows user to write the sms directly into the browser URL, using the format : https://myaccount.VoipCheap.com/clx/sendsms.php?username=xxxxx &password=xxxxxx&from=xxxxx&to=xxxxx&text=xxxxx

As for voice, in order of increasing call costs (see comment for cost table):
VoipCheap < Gizmo < Skype

Call cost is not the only decision factor, quality of service and ease-of-use also play a role too. I tend to launch Skype only when I intend to make a call - using GTalk as my preferred instant messaging client. Skype's unlimited EU landline calls require ~200 mins of calls each month to pay-off (vs PAYG).