gq{movement} to wrap text, or just gq while in visual mode. gqap will format the current paragraph. is.gd/7Onk

g< CTRL-G > to see technical information about the file, such as how many words are in it, or how many bytes it is. is.gd/9cYj

:%s/[.!?]\_s\+\a/\U&\E/g will uppercase the first letter of each sentence (except the very first one). is.gd/4xZW

:g/_pattern_/s/^/#/g will comment out lines containing _pattern_ (if '#' is your comment character/sequence) (via @vrybas_)

:vimgrep /stext/ **/*.txt | :copen searches for stext recursively in *.txt files and show results in separate window (via @danechkin)

%s/^ \ n/ / to delete all empty lines (remove spaces from command!) (via @marekkubis)

ggVG= will auto-format the entire document

In gvim, change the cursor depending on what mode you are in (normal, insert, etc): is.gd/9dq0

To search for a URL without backslashing, search backwards! Example: ?somestuff.com is.gd/4xZW

CTRL-w | and CTRL-W _ maximize your current split vertically and horizontally, respectively. CTRL-W = equalizes 'em.

Basic commands 'f' and 't' (like first and 'til) are very powerful. See :help t or :help f. is.gd/cCHy

:tabo will close all open tabs except the active one.

/\%>80v.\+ with search highlighting (:set hlsearch) will highlight any text after column 80. is.gd/8ekT

/< CTRL-r >< CTRL-w > will pull the word under the cursor into search. is.gd/7Onk

"+y to copy to the X11 (or Windows) clipboard. "+p to paste from it. is.gd/9dkI

ga will display the ASCII, hex, and octal value of the character under the cursor. is.gd/4xZW

[I (that's bracket-open, capital-i) shows lines containing the word under the cursor (via @nvie)

% matches brackets {} [] (), and with matchit.vim, also matches def/end, < ?php/?>, < p>/< /p>, etc. is.gd/N4W

:40,50m30 will move lines 40 through 50 to line 30. Most visual mode commands can be used w/ line numbers. is.gd/cCHy

In your ~/.vimrc, `set clipboard=unnamed`. Now all operations work with the OS clipboard. No need for "+, "* (via @panozzaj)