There is LyX that inkoia mentioned here that follows a WYSIWYM ("what you see
is what you mean") approach, where what shows up on the screen is an
approximation of what will show up on the page.
You can always open a text editor in one half of the screen and the document
viewer in the other half. GNOME's Document Viewer automatically reloads the
document when it is modified. With GNOME 3's window manager, just drag the
window to left/right border of the screen (and there are keyboard shortcuts
too) so that it takes half of the screen. I do not know if it is that simple
with other window manager.
Emacs can display PDF too. And it can vertically splits its window with C-x
3, so that the .tex is opened in the left buffer (with the default LaTeX mode
or AUCTeX) and the DVI in the right one. The DVI can automatically reload
the file when modified. Just add this line in ~/.emacs:
(add-hook 'doc-view-mode-hook 'auto-revert-mode)
