4 things i resisted using in hammer editor, and now i use them all the time:
Ctrl+e
Centers your 2d views on a selcted object(usually from the 3d view). Irreplaceable on a big/complex map.
QuickHide selected/unselected
Radio buttons next to the visgrouping ones, that temporarily hide selected or unselected objects. Again, great big/complex maps, specifically for quickly sorting out visual clutter in the 3d/orthograpic views, so you can more easily see what you are working on.
Paste Special
Pastes on the direct coordinates of the copied object. Great for reproducing multiple, off-grid stuff you could never otherwise align properly, because it's in-between gridpoints.
Cordon Tool and Cordon Compile
Isoloates an entire area from the editable/viewable area of the map(sort of like QuickHide, but for a whole area, not objects. Great for reducing your compile time for a specific area, and reducing editor lag for a busy map.
Actually wrote a tut on it
Nice tools captain, I only really use the hide tool. Never used any of the others, nice to know.
Edit: "plays with paste special" whoa wtf. Why don't we know about these tools.
I use the hide tool to assign the selected brush/ objects to a visgroup and name them. Sort of like layers in most modern modeling programs. Then I can hide and reveal them in the visgroup's window while I work.
Oh, right. The Core.
FML
Also, i never used paste special
Too bad after some point, no matter how good the PC is, it will still LAG.