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These are some of the keyboard shortcuts I use almost every time I work in AE. There are many, many more, but I think these are the one's worth memorizing first.

Importing files

apple-i Import a single file
apple-option-i Import multiple files
Return Rename a file or composition in the project window

The Timeline

p show the "position" attribute for the selected layer in the timeline.
t show the "opacity" attribute (think of 'transparency' to help you remember the shortcut) for the selected layer in the timeline.
r show the "rotation" attribute for the selected layer in the timeline.
m show the mask or masks attributes for the selected layer in the timeline.
f show the mask "feather" attribute for the selected layer in the timeline.
e show the attributes for any effects applied to the selected layer in the timeline.

Composition Windows

apple - + zoom in on the current composition window.
apple - - (minus) zoom out on the current composition window.
apple - option - + zoom in on the current composition without changing the window size.
apple - option - - (minus) zoom out on the current composition without changing the window size.