http_load runs multiple http fetches in parallel, to test the throughput of a web server. However unlike most such test clients, it runs in a single process, so it doesn't bog down the client machine. It can be configured to do https fetches as well.
You give it a file containing a list of URLs that may be fetched, a flag specifying how to start connections (either by rate or by number of simulated users), and a flag specifying when to quit (either after a given number of fetches or a given elapsed time). There are also optional flags for checksums, throttling, random jitter, and progress reports.
See the manual entry for more details.
% ./http_load -rate 5 -seconds 10 urls 49 fetches, 2 max parallel, 289884 bytes, in 10.0148 seconds 5916 mean bytes/connection 4.89274 fetches/sec, 28945.5 bytes/sec msecs/connect: 28.8932 mean, 44.243 max, 24.488 min msecs/first-response: 63.5362 mean, 81.624 max, 57.803 min HTTP response codes: code 200 -- 49