Not my exercise, but....

Trill using the first 4 frets of a string, one finger per fret. Trill between all finger pairs:

1,2
1,3
1,4
2,3
2,4
3,4

Builds strength and co-ordination. Worked for me. I used it for legato exercise, but you could pick it for a right hand workout as well.... no jokes please...