I'm thrilled and honored to say that Picker is now defunct--it has ascended to be a vanilla feature in 0.15 and above! (It's called "Pipette" and you can bind it in your Controls settings.)
In case you're playing .14.X or previous, see below...
I don't always have time to maintain Picker. If it's out of date, try here:
Picker adds a very simple feature missing from vanilla Factorio: the ability to middle-click an entity in the world to select the corresponding item from your inventory. It works just like the middle mouse button in Minecraft.
Middle-click an assembler to put assemblers in your hand. Middle-click a long inserter to select those. Most of the time when you're building things, it's in an area where you already have some of that thing built, so it's super convenient! It's like your whole inventory fits on your toolbar and you never have to remember which item is in which slot.
Not everyone likes the "Q piggybacking" functionality, so the 1.2.0 version now has two hotkeys:
Entity select (standard): Use the middle mouse button (by default) to select entities. Should be instantly familiar to anyone who's played Minecraft.
Entity select (Q piggyback): For people who like combining Picker with the Clear Cursor control. This is set to "Ctrl+Q" by default to disable it, but if you want to use it you should set it to Q.
Version 1.2.0 (22 Jul 2016)
+ There are now two separate keybinds available.
+ Piggyback mode (sharing the same key as the "clear cursor" command) is now a non-default option.
+ Standard mode is now the default, and is set to middle mouse button initially.
+ No longer tries to pick up coal, stone, iron ore, or copper ore from their deposits.
Version 1.1.1 (21 Jul 2016)
+ Now designed to share a key with the "clear cursor" command (Q by default).
+ Q over an entity to pick that item. Q over empty space to clear the cursor.
+ Does not interfere with wire running or rail laying.
Version 1.0.1 (18 Jul 2016)
+ Fixed problem with hotkey description introduced in 1.0.0 version.
Version 1.0.0 (18 Jul 2016)
+ You can now pick rails.
+ You can now pick an item if you currently have something in hand.