Why can't I open downloads from Google Chrome's download bar?
It's caused by a bug in 10.8.2. Occasionally revealing files in Finder stops working in all applications, and for example Archive Utility stops responding when extracting archives.

As a workaround, you can terminate appleventsd from Activity Monitor or with sudo killall -kill appleeventsd (without -kill the process is sent a TERM signal, which makes other processes stop responding). The issue will probably return within a few hours or days though.