I do not have complex enough projects at this time, so I only use the free timer from Toggl, but they do offered a paid version which does invoicing, I haven’t tried it out though. Toggle also has a widget. I just setup my own invoice template in InDesign and transfer the time stamps from Toggl to my invoice and export as a .pdf and email the client.
I appreciate all the input though, I am going to look into some of these others. The estimating features sound really trick.