Apache and Tomcat are not even close to distributed computing environments.
They're single-server environments, and neither is even particularly fast in that role.
They are both well known and well supported, however.
If your application is simple enough to run on a single server (no matter how many users, as long as there aren't too many at one time), then that type of solution is fine (and a lot easier to program).
If your application's processing gets more complex as more users log in (relatively few applications do this), then no number of instances of a single-server-model web-server will handle the load, and you'll have to accept harder programming in order to scale beyond a few hundred thousand users.

Bryan O'Neal wrote:
> Every time I run the analysis your better off writing for a
> distributable open source app engine, like Apache / Tomcat. And
> horizontally spanning as required on commodity hosts, like go daddy.
>
> On Tue, Jul 13, 2010 at 7:00 PM, Doc Media <doc_media@yahoo.com> wrote:
>> Anyone had experience (good or bad) with Google's App Engine? �A friend
>> of mine was looking to start a project, and we were discussing the finer
>> points of a regular hosting company versus something like App Engine.
>> Any insights would be helpful.
>>
>> - Scott