I don't have any online system yet, but I've been toying with spreadsheets and such. A database by vehicle/date/mileage/repair/projected repair ("exhaust will probably need to be done next spring" sort of thing)/maintenance/projected maintenance/notes would be ideal and hopefully not too tough to write up.

In the meantime, what I've always done is:

1. Notebook in each car with a bookmark. First 1/4 of book is events other than fuel or oil changes. Each entry has date, mileage, details of repair ("4 October 2008: New Tires at 125,567 miles, Kumho Ecsta XYZ 215/65TR15 on stock rims from Dunn Tire; receipt and warranty information in folder").
Second 3/4 of book is fuel and oil changes, noting date, mileage, trip miles, gallons and grade, MPG, station, city/location, notes ("4 October 2008, 125,545, trip 290 miles, 10.0 Gal-Regular, 29.0MPG, Mobil, Rte 417 Portville, NY, mixed driving around town, added 1 quart of Castrol 10W30"). I also make a note at the mileages where jobs were done that are described in the first 1/4 ("See New Tires at 125,567 miles").

2. I put all vehicle receipts in a folder in the house. As noted above, I write in my notes that the receipts/warranty info/whatever is in that folder. Also, any numbers or whatever that are important (road hazard warranty info, for example) I photocopy and keep in the car with the notebook.

3. I have a large whiteboard (made from a 4x8 melamine board)in the garage that has a grid:

Car: Miles/Date at OIL: COOLANT: ALIGNMENT: TUNE-UP: NOTES: *DUE*:
1998 Mustang
1988 300ZX
1987 F150
1980 Zephyr You get the idea so I'm not adding the miles/dates.
1965 122S
1983 GS650GL

This board is a mileage/date reminder only. No "10W-30" or "gap to 0.45" stuff. That's what the notebook is for. I just want a FAST glance to tell me if the vehicle I'm in is likely to need anything. *DUE* is the next expected repair and I write it down at the time of last service as I update the board. *DUE* is usually the next oil change mileage, but sometimes would be used with Notes: "Replace front control arm bushings when tires/alignment done next".
I don't cover brakes/tires as those are inspected regularly and mileage varies with brand, driving, etc.

A successful maintenance program would replace the board and the maintenance portion of the notebook; updating would be weekly (updating fuel added, etc). It would pop up reminder notes based on mileage and date.
It would NOT replace having the notebook in the glovebox although it would only be the "second half".
It would also tie into my calendar and complain annoyingly every Sunday if I didn't enter a mileage for every vehicle to make sure they were not forgotten, even if it wasn't actually driven.
Lastly, it could be searched so I could quickly figure out when a repair was last done and determine time between certain repairs.

Maybe someone with a bit more ambition than myself could whip something like this up?