Metronome
A metronome is an important tool for musicians. It is a small piece of equipment that produces a steady beat at varying speeds. Musicians practice with a metronome to learn the nuances of playing music in a steady beat (our natural tendencies are to speed up). Metronomes come in different sizes and shapes. I recommend a quartz metronome that is loud enough to be heard while playing but can also be used with an earpiece. Metronomes are used by everyone from beginners to professionals.
Tuners
Tuners are an important piece of equipment for musicians. They tell you if you are playing in or out of tune and how far in/out you are. They also aid our ears while tuning the actual instrument. There are several styles of tuners with a large price range. I recommend the Korg CA-30. Tuners are used by everyone from beginners to professionals.