In general, using the Shift key selects text, and the Ctrl key makes selecting words and moving your cursor go faster by selecting a word at a time.

Delete a whole word

Delete previous word: Ctrl+Backspace
Delete next word: Ctrl+Delete

Delete a whole chunk of text

Before deleting, use Ctrl+Shift+left or right arrow to select the text

Paste plain text (after copying)

In Microsoft Word, hitting Ctrl+Alt+V brings up the Paste Special dialog box. You can then paste the copied text as unformatted text.

In Gmail, use Ctrl+Shift+V to erase any text formatting.

Remove text formatting (after pasting)

Highlight the pasted text, then use Ctrl+Spacebar

Make text ALL CAPS

Highlight the text, then use Ctrl+Shift+A

For small caps, use Ctrl+Shift+K

Make subscript and superscript text

Subscript: Ctrl+=
Superscript: Ctrl+SHIFT+=

Get more text editing shortcuts here.

For keyboard shortcuts for email, check out Get Wired March 2014.