I don't know if this was ever posted(I couldn't find it in the archives) but if you hold the command key, when in a text document, and press any of the arrow keys, the cursor will move to that extreme: right arrow moves to end of line; left to begining of line; up to beginning of document; down to end of document. You can use this to select text as well by holding shift (it only works when going up or down).
[robg adds: I believe this will only work in Cocoa apps and certain Carbon apps, such as BBEdit. It doesn't work in Word, for example.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031125144942865