I didn't know at all apache ab sine yesterday when a guy who is working with us told me about this tool included in apache distribution.
Then I used with my site on localhost using this command line:

ab -n 9000 -c 900 localhost:8080/index.php

I got this results:

 Completed 900 requests
 Completed 1800 requests
 Completed 2700 requests
 Completed 3600 requests
 Completed 4500 requests
 Completed 5400 requests
 Completed 6300 requests
 Completed 7200 requests
 Completed 8100 requests
 Finished 9000 requests
 
 
 Server Software:        Apache/2.0.55
 Server Hostname:        localhost
 Server Port:            8080
 
 Document Path:          /index.php
 Document Length:        37949 bytes
 
 Concurrency Level:      10
 Time taken for tests:   407.890625 seconds
 Complete requests:      9000
 Failed requests:        25
    (Connect: 0, Length: 25, Exceptions: 0)
 Write errors:           0
 Total transferred:      342093375 bytes
 HTML transferred:       340598850 bytes
 Requests per second:    22.06 [#/sec] (mean)
 Time per request:       453.212 [ms] (mean)
 Time per request:       45.321 [ms] (mean, across all concurrent requests)
 Transfer rate:          819.03 [Kbytes/sec] received
 
 Connection Times (ms)
               min  mean[+/-sd] median   max
 Connect:        0    0   2.7      0      78
 Processing:   140  452 217.8    359    2265
 Waiting:       31  402 194.0    328    2218
 Total:        140  452 218.0    359    2265
 
 Percentage of the requests served within a certain time (ms)
   50%    359
   66%    390
   75%    421
   80%    515
   90%    750
   95%    968
   98%   1156
   99%   1281
  100%   2265 (longest request)

Mmhh.. 25 errors with 9000 connection and 900 concurrency level?
I must admit I never used stress test tool at all by my own, just one time I see it used when working for a big java based project.
Do you use some kind of benchmarking tool?