Siege is described as 'regression test and benchmark utility. It can stress test a single URL with a user defined number of simulated users, or it can read many URLs into memory and stress them simultaneously'. There are five alternatives to Siege for a variety of platforms, including Linux, Mac, Windows, Self-Hosted solutions and Python. The best alternative is Apache JMeter, which is both free and Open Source. Other great apps like Siege are locust (Free, Open Source), hey (Free, Open Source), Apache Benchmark (Free, Open Source) and wrk (Free, Open Source).
Apache JMeter is an open source Java application, designed to load test functional behavior and measure performance. Originally designed for testing Web Applications, it has since expanded to other test functions as well.
ab is a tool for benchmarking your web server. It is designed to give you an impression of how your current web server installation performs. This especially shows you how many requests per second your web server installation is capable of serving.
wrk is a modern HTTP benchmarking tool capable of generating significant load when run on a single multi-core CPU. It combines a multithreaded design with scalable event notification systems such as epoll and kqueue.