A metal ruler is useful as a tool for framing because measuring tapes aren't always as fine as you may need to measure -- especially for smaller frames. The ruler is also useful as a means to trace straight guidelines for cutting your wood. The hard edges of metal rulers are also handy to scrape away excess glue once the frame has been completed.
A staple of any tool bench, your measuring tape is your primary measuring tool when cutting your wood to size and trimming off rough ends. The benefit to having a measuring tape as well as a ruler is the greater length that a measuring tape can measure. This allows you the option of constructing larger or thicker frames than may be possible with a ruler alone.
Though any square object could be used, a drafting triangle is recommended when setting the miter fence on your table saw to 90 degrees. It is very important to set your saw to 90 degrees exactly, as any variation will cause your frame to hang asymmetrically. A drafting triangle is also convenient as it is light and easy to store.
A pencil is useful for framing because you will need to mark every cut before you make it. A successful frame is a precise frame, and the easiest way to ensure precision when you're cutting is to know exactly where to cut. A pencil is the best choice since inks or markers can bleed across the grain of the wood into areas that are visible when looking at the front of the frame.