The Diccionario panhispánico de dudas (DPD) calls ello the neutral form of the the tonic third-person singular pronoun and mentions that The syntactical properties of ello have diminished and some normal uses in previous eras have disappeared. Nevertheless, the DPD lists the following current usage cases:
Ello as the terminus of a preposition:
Ana trató de localizarlos en cuanto tuvo fuerzas para ello.
Which roughtly translated means:
Ana tried to find them as soon as she had the strength for it.
It being Ana's act of trying to find them.
Ello as an indirect compliment:
Los hombres superiores son virtuosos y a ello debían también sus dones.
Which roughly translates to
Superior men are virtuous and to that they owed their gifts.
Where that refers to the fact that superior men are virtuous.
As a subject (rare, usually only in written form):
Typically equivalent to lo cual:
Sé que fue casi atroz mientras duró. Ello no significa que su relato pueda conmover a un tercero.
Which roughly translates to
I know it was almost atrocious while it lasted. This does not mean that its telling could affect a third person.
Where this refers to whatever act was almost atrocious.
With the verb ser:
Solo quiero consolarte, hasta donde ello es posible.
Which translates to
I only want to comfort you, as far as it is possible.
Where it refers to the act of comforting.