What are some alternatives to Quicksilver?
Alfred is the one that meets pretty much all your needs.


Launching applications, preference panels and apple scripts.
Web search, and search within bookmarks of my favorite browser.
Able to perform actions or operations on files (copy, move, reveal in Finder). paid version only
Custom web searches.
Free.
Plugin infrastructure.
A healthy community of developers actively maintaining the application.
Not too many bugs