Following @bmikes suggestion I am listing the apps I have looked at so far. I must emphasize that none of them really solves the problem stated above perfectly.
Here's what I have looked at so far. Note: My first draft of this post included links to all the apps but AskDifferent won't let me post more than two links. Find the same list in this Google Doc with working links to all the apps. Sorry for the inconvenience.
- On the Job looks great, but the developer seems to have abandoned it. At least he does not really fix bugs and is very slow to respond to his user base.
- Timings light-weight but the UI to create clients, projects and tasks can be confusing. I am testing it right now. A friend uses it and says it crashes sometimes though it has not happended to me yet.
- Harvest looks good, it does have a desktop client itself and an API that is used by lots of apps. But it is subscription-only which is a no-go for me and costs at least $12 per month.
- The same goes for Toggl: looks good, but is subscription-based and consts $5 per month
- iBiz and timeEdition have been discontinued
- Klok is an Adobe Air app. The UI feels weird. Small typography, limited support for keyboard shortcuts and complex interface.
- OfficeTime feels like the best match so far but the UI is very cluttered (see this screenshot of the Generate Bill dialog as an example). It feels a bit like a Windows app that has been ported to Mac but I am not sure if that's true.
All I want is a pay-once, stand-alone, well-supported and nicely designed time-tracking app. Am I too demanding? Or missing a good product?