Salat al-Hajah — traditionally associated with asking Allah for assistance.
Salat al-Tasbih — a disputed/variously graded voluntary prayer; not universally practiced.
Tahiyyat al-Wudu — 2 rak'ahs after making wudu, as a voluntary prayer.
A reference, not a fatwa — exact rak'ah counts and practices for some voluntary prayers vary between schools of thought; ask a local scholar for anything specific to your situation.