R-Trees

I'm looking to do some spatial searching and joining of data but  
without an spatial index it seems to take a considerable amount of  
time. For example, I have user data with locations and would like to  
determine what zip code code they are in thereby requiring a lot of  
point-in-polygon calls. Is there either a better way to do this or an  
r-tree implementation somewhere (searching revealed no)? I'm beginning  
to implement an r-tree index but before I invest too much time want to  
ensure I'm not duplicating someone else's work.

Thanks,
Jason