I use a spreadsheet, but on Google Docs, not Excel.
With Google Docs, I can add in data from my cell phone, so I don't have to write things down and bring them in to enter later on my computer. But I still have computer access too.
The workbook mostly has two sheets for each vehicle.
One is a log, of date, mileage, work performed, and notes
The other is a scratch page of part numbers, sources and prices, when I've replaced them, etc. Also, information on any codes I've deciphered (maintenance minder, MIL/DTC, etc.)
I have one sheet that has the tire sizes for all of my vehicles, one that has wiper sizes, one that has oil filter part numbers, and one that has bulbs.