What are all the different third-party accessories that I can get for an RCX?
My favorite third party accessory is the Playstation 2 controller interface for the NXT.
Also you can check out their entire listing here:
They have some really great third party accessories that are not that expensive.
About 8 to 10 years ago there was more RCX-compatible sensors makers on the web. Today, I don't think we'll find them unless we dig into the Internet Archive.
It looks like Techno-Stuff is still making some RCX-compatible components. Also, Mindsensors was making RCX-compatible components also but I couldn't find links to them through their website main page (It might be discontinued). You can probably still buy some through eBay. Here are two of their RCX compatible products: