Found this key fob from a post on Audizine and purchased off eBay. Works great except the trunk button doesn't do anything. I'm just glad i finally have a spare set of keys.... only took me 9 years to get another fob. Just for the record, the B5 line does not use a "immobilizer chip", so if you get one of these, don't get suckered into paying for that to be "programmed". The locksmith I went to get the key cut was convinced it would not work, but I got to show him that he was wrong.
And a little videos to show it really works:
Steps to Program your Key Fob for Wireless Entry
Source: Audizine (link)
Before you start, make sure you have at least two keys that can manually unlock/lock your car. You can not program your key fob with only one key.
Clear your Key Fob Memory Positions
Our B5 S4 can store up to four key fobs for wireless entry, but to simplify the process of programming it best to start with a clean slate.
Re-programming your New and Old Key Fob.
When I first attempted this I noticed I could only get one remote working at a time. That is when I realized I was saving one key fob in the same memory slot. Take careful attention to step #3 and #8 as you are selecting which "memory slot" you are recording the key fob too. The number of times you press the unlock button will instructed which memory slot is used. 3 presses will go to memory slot #3 or a single press would go to memory slot #1. Remember, there is only 4 memory slots, so don't go crazy and press the unlock 5 or more times as that will clear all key fobs and you will have to start over.
New picture with Audi logo