What are all the different third-party accessories that I can get for an RCX?
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:


UltraSonic Proximity Detector v2
Motor powered Sharp GP2D12 infrared distance sensor interface