Is there a service I can use to test the reliability of my server? I need to check if my server can handle a large traffic spike.
// Server info:
Server runs Nginx 0.7.65 on CentOS 5.3, serving only static files.
Freshmeat.com is another place to search for load tools.
Note that you will want to test both continuous load capacity, and the ability to handle and recover from spikes. Sudden increases in traffic (even if that peak is below the continuous load max) can cause your server to fall over and not recover.
You could use JMeter to put a load on your server and see how it performs.
I've been following a project call multi-mechanize for load testing. They have some instructions on getting it running in rackspace's cloud. Since the rackspace cloud is pay for what you use you can launch an instance or a few instances that has high bandwidth and run a realistic test against your server without breaking the bank.