+1 for Exifer. I've been using that for years. The IPTC and EXIF data is (generally) standardized, and any compliant program should be able to share the data.
I used to use IMatch. The software is incredibly advanced, the UI is well-thought-out, allows access to a centralized database on the server, and even has a scripting model. However, the upgrade process was so onerous that I had to drop out. One had to download and install the trial, then find your password to download the registered version, find the registration code and install it over the top, then find different passwords and registration codes for the "service packs" to be installed over the top (or at least that's how I remember it). I grew so confused, and had such trouble tracking all the (changing with each version!) passwords and codes, that I could no longer install it.
Now I just use ACDSee, which isn't as capable, but at least it doesn't put the user through such torture.