Alright, here goes.

The major players in the rally odometers are Alfa, Timewise, Terratrip, Brantz, and Coralba.

Alfa: http://www.rally.cc/

These are primarily TSD boxes, though the Pro is a more basic unit that's somewhat suitable for performance events.

Timewise: http://home.comcast.net/~timewise1/

The only Timewise units I've seen in use are their TSD boxes, again not all that suitable for performance rally. These are the gold standard of TSD computers.

Terratrip: http://www.terratrip.com/home.htm

The terratrip boxes are built for performance. For what they are, they do the job. I really don't like the display on them though, because they're often hard to read depending on the light conditions, etc.

Brantz: http://www.lugo.pt/ingles/ing-brantz.htm

The Brantz is in my opinion one of the easiest to read at speed, in bright sun, etc. It's more of a performance box, though the one model does have average speed, and the Laser has lots of fancy stuff you'd likely not use. The major issue with Brantz computers is I don't think there's anywhere in Canada you can get one new anymore.

Coralba: http://www.coralba.com/rally/readone.php?categoryID=15&id=11

The coralba is the gold standard of the performance boxes. You often see a C-Giant in the top open class cars, and there's a reason for that. They're big $ though.

When it comes down to it, you're probably best to go simple and get a used Terratrip 202 or 303. With stagenotes and recce becoming ever more popular, the use of an odometer is becoming less and less critical. I've never actually ended up using one on any of the performance events I've co-driven, but I've only run on notes. If you're running on a routebook, you will need it to call down distances to cautions, etc. If you're on notes, as long as you have geographical references to get yourself back on track if you get lost in the notes, you don't even need an odo except for transits, and even that isn't all that hard to do without it.