math-modesymbols
Essentially, I am trying to typeset a symbol in math mode that looks like this: 
. Is there any way I can do this easily with a package or do I have to fiddle around with my own commands?
I tried vdash^*_M, but this leads to a vdash that is too short and with the * and M misplaced.
Edit: Following TH's advice, this would be a better command to use:
\newcommand{\trel}{\mathrel{\mathsurround=0pt \mbox{\raisebox{1.2pt}{\tiny \textbf{\textbar }}$\rhd$}}}
(I decided to explicitly say \mathsurround=0pt rather than just calling \m@th because that way, if Mike wants to paste this into the top of his document, he won't have to call \makeatletter)
Original: I don't know what built-in symbols would correspond to the one you're describing, but if you're just trying to get something done you could use this:
\newcommand{\trel}{\mbox{\raisebox{1.2pt}{\tiny \bf \textbar }$\rhd$}}
(and then use \trel every time you want to use this relation).
It's a huge hack, but, like I said, it might be enough to get you by.
You have known Morten Høgholm's mathtools, then you can try his breqn package:
\documentclass{article}
\usepackage{breqn}
\begin{document}
\hsize=12cm
\begin{dmath}
f(x) = x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x = x*c
\end{dmath}
\hsize=6cm
\begin{dmath}
f(x) = x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x = x*c
\end{dmath}
\hsize=3cm
\begin{dmath}
f(x) = x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x = x*c
\end{dmath}
\end{document}
Best Answer
You can use the
turnstilepackage:You can also look at https://tex.stackexchange.com/a/29091/4427 for a slightly different realization.