depends on wether you want to make a web-based game (as in browser game, eg
http://www.ogame.org/) or a in-browser game (like the flashgames on
http://www.kongregate.com/) or a online-multiplayer game.
in any case, this is the rundown:
* flash is the de-facto industry standard if you want to write a in-browser game.
lots of online documentation, install-base is huge, etc. silverlight is mostly used for microsofts own stuff. i think there was one java based game out-there, but that's about it.
i tried playing around with hotruby, but never got the compiling right.
for online multiplayer games (that need to be downloaded) you can use gosu, as soon as it includes the ruby standard network libraries.