Hopefully there is a markdown-mode for Emacs, but it isn't packaged (yet). After a quick ITP (Debian bug #435485), and having having asked around, it turned out that pushing it into the emacs-goodies-el package would be the right thing.
It was sufficient to follow the instructions in 00AddingFiles to get
an updated package, with a functional and documented (README.Debian,
debian/control, Texinfo manual) markdown-mode for at least both
emacs21 and emacs22, hence the wishlist bug (Debian bug #435631).
Having a look at the other wishlist bugs, some might be of some interest, and a patch could be forged to make them available and ease the packagers' workload:
showspace.el (Debian bug #422876): highlight various spaces, tabs, etc.
graphviz-dot-mode.el (Debian bug #428601): major mode for the graphviz dot format. Yay!
Other wishlist bugs, which have to be inspected a bit: Debian bug #336398, Debian bug #314664, Debian bug #228550. (They might have seen no answer in 2 years, or be waiting for an answer from upstream, etc.)
Nice modes already included in emacs-goodies-el:
color-select: nice colors include Arjen, Billw, Calm Forest, Cheap Goldenrod, Clarity and Beauty, Comidia, Dark Laptop, TTY Dark, White on Black.