jmc

The units digit of $18^6$ is the same as in $8^6$. There are several ways we could go about finding that units digit, but notice that $8^6=2^{18}$. It's easy to find the pattern of units digits for powers of 2: $$\begin{array}{rl}{2}^1& =2\\ 2^2& =4\\ 2^3& =8\\ 2^4& =16\\ 2^5& =32\end{array}$$Using this pattern, the units digit is found to be $\boxed{4}$.