A useful shortcut/trigger for working with active files in Quicksilver is the "Current Selection" trigger that opens the Quicksilver command window with the files currently selected in Finder.app.
Unfortunately, Apple's implementation for this has broken in recent versions of OSX, but there are two workarounds until Apple fix the problem (unlikely ever to happen).
Also see Grab 'n Drop.
There are two proxy objects that can grab the current Finder selection: "Finder Selection" and "Current Selection".
"Current Selection" also works on other kinds of selections, like a URL highlighted in a text editor. Thus, you'll probably want to use "Current Selection" unless you're either making a trigger that should only work on a file/folder, or need to get the Finder Selection while another program is in focus.
Beta 72 now includes a default trigger for this, called "Command Window with Selection". (Under Quicksilver preferences > Triggers > Quicksilver, called "Command Window with Selection".)
Send to Quicksilver (Service)
Services can be a bit tricky. Any OSX application can register a "service" which is available from the application menu under "Services". You can assign shortcuts to them and use them much like you would use a Quicksilver trigger. Aside: Quicksilver can use services from other applications using the Service Menu Module.
Quicksilver has a built in service called "Send to Quicksilver", functioning similarly to the "Current Selection" proxy object.
This service has the keyboard shortcut ⌘⎋, which is the same as the "Command Window with Selection" trigger built into Quicksilver. If you wish to use the service separately from the "Command Window with Selection" trigger, then you should change the keyboard shortcut for the trigger in the Trigger preferences.
- The proxy object suffers from a serious, well-known bug in recent versions of OSX. Sometimes it returns the wrong object, usually the previous Finder selection from a different window. (Imagine what could happen if you create a trigger for an irreversible action like "Delete (Erase)".) There is no good known workaround, so if you'd like this to be fixed, please create one more bug report to tell Apple to fix this: 
- "Send to Quicksilver (Service)" cannot be used to make triggers, and doesn't work on the current Finder window if there's nothing selected.
|Behaviour||Proxy Object||Send to Quicksilver|
|Always returns the correct selection from Finder.||✗||√|
|Works in Finder with a file/folder selected||√||√|
|Works in Finder if there is no file/folder selected (returns current window's folder)||√||✗|
|Works in Finder with column mode||√||✗|
|Works on selections in other programs||√ (Use "Current Selection")||√|
|Can be used with triggers||√||✗|
|Does *not* have a tiny proxy resolution delay (used to be 0.5s, soon to be 0.1s)||✗||√|
|Is *not* cached for 3.0s (where using a cached proxy object again within 3.0s will not refresh the object)||✗ (same for all proxy objects)||√|
|Does *not* change application focus to Quicksilver.app (only relevant if Quicksilver is in the Dock)||√||✗|
(Note: This table is organized so that all "good" behaviour is "√" and all "bad" behvaiour is "✗".)
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