Quicksilver is perhaps the most modular of the launchers, with dozens of free plugins available to add features — like image and text manipulation — and allow integration with apps like Evernote, Google Chrome, iTunes, and Safari. It's also the most explicit about the separation between the item found (an app, a document, a contact, etc.) and the actions you can perform on it. The default action is nearly always Open (accomplished by pressing Return), but you can instead press Tab and view a catalog of other actions appropriate for the selected item.
User-friendliness and performance
While all four of these launchers are similar at a high level, each one has its own spin on the features it offers. Given my own tastes and mindset, I find LaunchBar to be the most straightforward and best designed of the bunch. You can use it almost immediately with barely any configuration, though it's easy to customize if you like. And it's both extremely fast and predictable, largely because it uses its own index rather than relying on Spotlight's index. (LaunchBar is also good at detecting multi-word names and camel case words, so if you type "FT," FaceTime is likely to be at the top of the list.)
Although Spotlight's usability is good and its range of features is impressive, its speed is not. Even on a fast Mac, it may take two or three seconds for the full list to appear, and during that time, elements in the list may rearrange themselves, increasing the likelihood that you'll select the wrong thing.
Alfred relies on Spotlight for everything except for apps, preferences, contacts, and Safari bookmarks, so enabling documents, folders, and other items in Alfred's list can slow it down, although you can work around this by typing "open" to start a search for these items. Alfred depends on the user knowing such keywords for various tasks — so there is a bit of a learning curve involved — but on the whole, it's a friendly and well-thought-out app.
Quicksilver uses its own index, too, but its performance slowed to a crawl when asked to index a big batch of files (such as everything in my user Documents folder) — and was even worse during its periodic index rescans. Although Quicksilver is simple to use for most tasks, it can be challenging to find your way around its preferences. And some plugins, like 1Password, are currently far out of date.
Butler frequently left me scratching my head. I didn't have any particular problems with performance, although it continuously searches when indexing large folders (such as your Documents folder) — descending into all the subfolders, no matter how deeply nested — just to show off its usefulness. My bigger complaint about Butler is that it's cluttered and oddly organized — it adds three system-wide menus in addition to the pop-up search window, and the preferences are often inscrutable.
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