Spotlight opens files, does math, and gives definitions. Quicksilver does everything. It's based on an object/action system where you pick an object and do something to/with it.
There are all sorts of actions, from relaunching apps, deleting, emailing, or moving files, running scripts (I use get ip frequently), reading text, etc. It also gives fast access to some non-file things, like address book cards (right arrow to drill in to contact information, which let's you make new emails).
There are addons for a lot of things like bookmarks for various browsers, mouse gesture triggers, Cyberduck bookmarks, etc. It's fantastic if you prefer a keyboard over mousing.
There are all sorts of actions, from relaunching apps, deleting, emailing, or moving files, running scripts (I use get ip frequently), reading text, etc. It also gives fast access to some non-file things, like address book cards (right arrow to drill in to contact information, which let's you make new emails).
There are addons for a lot of things like bookmarks for various browsers, mouse gesture triggers, Cyberduck bookmarks, etc. It's fantastic if you prefer a keyboard over mousing.