In order to play Flexible Survival, you must either use the in-built MU* client on the webpage (requires Flash Player) or download one from a wide selection of MU* clients (preferred). The following list takes inspiration from a Wikipedia comparison listing; however, it truncates the list to trusted software and links directly to clients' websites.
The operating systems a client can run on.
Information about what protocols the clients support.
SimpleMU is only safely available via archive.org as the official site has been taken down.
The UI Guides section contains some useful configuration tips for clients contributed by players. If you know how to set up a client that isn't listed there, add an entry for it to help other players!