Here's a little script to post the currently selected headline in NetNewsWire (my favourite Mac OS X newsfeed reader) to your instapaper account.
You need to create an instapaper account before using this the first time. The script is not very good about errors, so just correct the issue and retry the script. It creates a short summary by "un-HTML'ing" the description, but it does this naively (using a long sed invocation), so it's restricted to a subset of HTML entities -- basically common Western ones.
To install this, select and copy the script from the box, paste it into the AppleScript Script Editor and drop it into NNW's script folder.