For many years, I just used standard tuning on my guitar when I played slide. E-A-D-G-B-E. I mainly used the slide to play the slide-lead guitar part of a song, playing chords during the rhythm parts with my other three fingers or just putting on the slide right before the lead.

Using open tunings for the slide can be very useful for playing rhythm as well as solo parts. An example of a widely used open tuning is the open G tuning, D-G-D-G-B-D. With your guitar tuned to an open chord or an open tuning, anywhere directly over a fretbar that you place your slide will be a chord.

In open G tuning, open is a G chord, of course.

1st Fret - G#
2nd Fret - A
3rd Fret - A#
4th Fret - B
5th Fret - C

6th Fret - C#
7th Fret - D
8th Fret - D#
9th Fret - E
10th Fret - F
11th Fret - F#

And it starts all over again at the 12th fret (G), repeating the above chords.

A song that uses open G tuning in a slide context is the Black Crowes “Twice As Hard”. Other example include “Honky Tonk Women” and “Start Me Up” by the Rolling Stones.