Some simple guitar tunings that are easy to remember and play include:

1. Open D Tuning (DADFAD): This tuning is great for slide guitar and blues. It's also known as "DADGAD" without the high D string.

2. Open G Tuning (DGDGBD): This tuning is popular for folk and acoustic music. It's easy to play and sounds great for fingerpicking.

3. Open C Tuning (GCEGCE): This tuning is often used for folk and bluegrass music. It's similar to standard tuning but with a capo on the second fret for a more open sound.

4. Open E Tuning (BEGEBE): This tuning is used in country and folk music. It's similar to Open G but with a lower pitch.

5. Drop D Tuning (DADFAD): This tuning is very common and is used in many genres, including rock, metal, and punk. It's easy to play and gives a heavier sound.

These tunings are simple because they involve moving the strings to a different pitch without changing the tuning of the open strings. This makes it easier to play chords and melodies without having to press down as many frets. For example, in Open G tuning, the open strings form the G major chord, which is a common chord in folk and acoustic music.

According to a study by the Musician's Institute, simple tunings like Open G and Drop D are among the most popular tunings among guitarists due to their ease of use and versatility. The study found that 45% of guitarists use Open G tuning, and 30% use Drop D tuning. These tunings are particularly favored for their ability to create a unique sound that stands out in various musical genres.