Jaduka have an interesting API based offering that lets you integrate a "click to initiate call" system to your website.
You basically choose a button, decide on what number to point it at and off you go. Setup apparently takes around 5 minutes, so definitely worth a look.
Hat tip to
Thomas Howe for highlighting it. If Thomas isn't in your feedreader yet, put him in now. One of the better quality blogs around in VoIP (by some margin).