Sally rolls an $8$-sided die with faces numbered $1$ through $8$. Compute the probability that she gets a power of $2$.
1. Identify the total number of possible outcomes when rolling an 8-sided die. Since the die has faces numbered from 1 to 8, there are 8 possible outcomes.
   \[
   \text{Total outcomes} = 8
   \]

2. Determine which numbers on the die are powers of 2. The powers of 2 within the range from 1 to 8 are:
   \[
   2^0 = 1, \quad 2^1 = 2, \quad 2^2 = 4, \quad 2^3 = 8
   \]
   Therefore, the numbers 1, 2, 4, and 8 are the powers of 2 on the die.

3. Count the number of favorable outcomes. There are 4 favorable outcomes (1, 2, 4, and 8).

4. Calculate the probability of rolling a power of 2 by dividing the number of favorable outcomes by the total number of possible outcomes:
   \[
   \text{Probability} = \frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}} = \frac{4}{8} = \frac{1}{2}
   \]

The final answer is \(\boxed{\frac{1}{2}}\).