Well, the title says it all. Also, it would be interesting to hear numbers in other sports as well.
Just to help you narrow your question, in the US between 1973 and 1995, there were 88 deaths among young baseball players(amateur) from injuries suffered in a game. http://www.kidsource.com/CPSC/baseball.6.10.html
Without thinking about it too hard, the total number of deaths in major league baseball games has to be under 10.
As to boxing deaths, here’s a listing of all http://ejmas.com/jcs/jcsart_svinth_c_0700.htm
I knew that Ray Chapman was the only famous Major Baseball player killed, but I guess he was the only one.
Wow. 40 deaths since 1/1/2000.
How many people died in boxing, well eventually all of them. OK I know you meant while boxig, but I just wanted to be the first to say that. 
According to this site:
1931-2003
Football* deaths total:1629
average: 22 per yr
According to the site** given by samclem and Reeder:
1931-2003
Boxing deaths total: 695
average: 9.6 per yr
*American Football
**records go back to 1741
I like the fact that there exists a “Journal Of Combative Sport.”
This article mentions at the end about some of the reasons for fatalities. One reason that’s not mentioned is the corner letting their guy (meal ticket) keep going out when he’s completely outmatched and absorbing a terrific beating. So many fighters’ fatal injuries happen not from a single punch, but from the combined beating sustained thru many rounds.
Two other people who did not suffer fatalities, but career-ending injuries:
Carmine Vingo. Vingo, much taller than “The Rock” and a fearsome puncher, was up against Rocky Marciano, who was on his way up at the time. However, Marciano caught Vingo in about the sixth round. Vingo was out on his feet, and The Rock kept throwing leather. By the time Vingo fell to the canvas, it was obvious something was wrong. Vingo almost died that night, but eventually made a complete recovery. As recently as a few years ago, he was still alive, while Marciano was the one who had the shortened life as the result of a plane crash in 1969.
Greg Page. Former heavyweight champ (of some organization). A few years ago, he was fighting for a $1500 purse. Here is his story:
http://www.counterpunch.org/johnson01102004.html
BTW, I saw the Emiliano Valdez fight, who is mentioned in that list earlier in the thread. That fight was filled with fouls on both fighters’ parts. In the last round, Valdez quit fighting and motioned to the referee as if to say “That’s all.” He dropped his guard, and when he did, Reid landed two solid shots square on the head. Valdez never woke up, dying 13 months later. What I didn’t know, though was that just a few weeks before that, that Valdez had killed a man sparring.
All well and good, but whats the number of participants in football vs boxing? I’m sure football is far more popular.