This could get ugly. Already we have vi v. emacs (I mentioned emacs just because I like the tables in that mode - I personally use textmate), reStructured text (controversial because of the use of white space, ie. the great perl v. python debate), markdown (useful for html conversion and the decent syntax coloring in textmate but not really very good for structured data in any sense other than just outlining), xml is readable enough v. xml is evil (my bad), etc.

Rather than rehash all that, maybe a link to http://c2.com/cgi/wiki would suffice.

gte910h: For time and billing, I use paper. Not just because it's portable but because I need to see my day as a timeline, dayplanner fashion. Also because relying on the computer to add up my time makes mistakes more likely, in my experience. It's a hassle to convert your physical timeslips into a spreadsheet, or whatever, but forcing yourself to do this makes for accuracy and a better sense of how you're spending your day.

Not trying to talk you into anything - it sounds like you're comfortable with your system. But my experience is that what you gain from computer timekeeping is mainly an illusory sense that you are saving time on the back end (adding up the time, which you should be doing manually and attentively to ensure accuracy), while you make more work for yourself on the capture end (entering data as opposed to just making a mark in your planner).