Vim: remap key to toggle line numbering

In your .vimrc, add this:

set number
nnoremap <F2> :set nonumber!<CR>

Then pressing F2 will toggle line numbering.

This is one method:

map <silent> <F2> :if &number <Bar>
    \set nonumber <Bar>
        \else <Bar>
    \set number <Bar>
        \endif<cr>

(this one is nice 'cause I usually put foldcolumn in there as well)

This is another:

map <silent> <F2> :set invnumber<cr>

(direct method)

This is what I use (with a different key binding):

nmap <f2> :set number! number?<cr>

The "number!" toggles the setting and "number?" reports the state.

nmap <silent> <F11> :exec &nu==&rnu? "se nu!" : "se rnu!"<CR>

In new vim you can set both relative number and number at once, this way:

set nu rnu