I need a convenient calculator to figure how long a certain dollar
amount will last with a) regular monthly withdrawals and b) a fixed
rate of return on the balance.

In other words, how long (for example) will $100000 last if it's
earning 4.5% and I withdraw $5000 per month?  ... at 5%? or at
$4000.00 per mo.? etc.

This is basically what a credit card pay-off calculator will determine
(just viewed from the other side).  USA Today provides one at
http://www.calcbuilder.com/cgi-bin/calcs/CRE4.cgi/usatoday
Just enter the amount into "Amount Now Owed", zero out "Future Monthly
Charges" (this would be where you would enter regular additions to
your starting figure), Set "Future Monthly Payments" to whatever you
want to withdraw each month (the $5000 in your example), set the
"Annual Rate" to whatever return you're getting (4.5%), and zero out
the "Annual Fee".  Leave the Months desired as is.  Set "Future Rate
Changes" to no change unless you expect your return to be changing. 
Doing all of this with your given example would yield the result
"Otherwise, given your anticipated payments of $5,000, you'll repay
your outstanding balance in 21 months."  21 months is the answer
you're looking for.  Plugging in other numbers will get you the
appropriate answers as well.

Hope this helps.
-Brad (who wishes he were an official answerer and could get paid for
this... :) )

http://www.interest.com/hugh/calc/rdur.cgi

This is a really simple calculator.  I did a GOOGLE search for
"financial calculator annuity" & got a bunch of hits.

Thank you both for the helpful comments. 
And thank you, Google, for having the foresight to establish this
truly unique and valuable forum for all of us who wander in the
darkness, looking for the light.