iFiction.org lets you play many text adventure games over the web, using an exceptionally clever PHP-based z-code interpreter. A real z-code interpreter allows for fancy things like timing tricks, but since most text adventure games are basic command-and-response, this site takes commands with a web form and renders the next screen as a web page. Despite apparent limitations, however, this funky z-machine plays "z-machine abuse" games rather well.

Hundreds of fun games online, too, including every game from every online IF competition, ever. An excellent way to try them out without bothering with interpreters and game files.