Practical applications of disjoint set datastructure
Maze generation (using a modified Kruskal's algorithm)
Tarjan's off-line least common ancestors algorithm 
Connected component labeling
Online maintenance of biconnected components
Validation Hindley–Milner rules
Computing the winner of Havannah board game (see Efficient Playouts for the
Havannah Abstract Board Game)
Alias analysis
Used in construction of contour trees (see Laying the Foundations for an Advanced
Visualization System in O'Caml and Multi-Resolution computation and presentation of Contour Trees)