Can anyone suggest an invoicing app with a simple and clean UI for Windows? It should be low priced (free is OK too).

For Mac OS there seems to be quite a few such apps available. Not including anything remotely similar to QuickBooks. Doesn't even need time tracking/counter features. Mainly just something to create the invoices and track them, being able to mark them paid, send copies out via email, print, etc...

Examples of Mac software:

Profit Train (Formerly Billable): http://clickablebliss.com/profittrain/
Billings: http://www.marketcircle.com/billings/ (ok, this one is a usability nightmare even though it looks pretty enough in screen shots.