Yantra's a visible source of devotion

Yantra's as a way to give effort to religion

The Gayatri Yantra, the most profound visible religious item.

To use it, you might want to place it as art on a wall.
It grants the owner a good and prosper life.
Gayatri as the wife of Brahma

The mantra on which the yantra is based

Om Bhur bhuvah svaha, Tat savitur varenyam
bhargo devasya dhimahi
dhiyo yonah prachodayat

The Goddess Gayatri has five heads:
four depicting the four Vedas and one depicting the Almighty of the Universe.
Her Yantra is a sign of auspiciousness and upon praying regularly,
you can be blessed with good health, wealth, prosperity and peace

cited from the website astroved.com

Vishnu Yantra
Vishnu as the grand support of this universe.
He as the upholder of the reality

the mantra on which the yantra is based

Om narayanaya vidhmahe
Vasudevaya dhimahi
Tanno Vishnu prachodayat

Maha Vishnu is the protector of this Universe;
he guards all the living beings on this earth plane.
When you worship him, you will be gifted with long life,
prosperity, peace, happiness and success.
Impediments will no longer stand on your way of achievements.
His Yantra is energized with his power to rejuvenate your spirits and self confidence.
Cited from the description on astroved.com

Laksmi Yantra
Laksmi is the goddess of fortune and wealth.
Use the Yantra to brighten your life.
Sometimes a yantra is to expensive to buy,
So this one comes with the artprint for 30 dollars or a free downloadable version.
If you download it, you can print it yourself.

Laksmi also is the consort of Vishnu.

Sri Yantra, most used one
The Lord named by Sri, is the yantra being the most widely used yantra,
invoked to give life prosperity and a glance of success.
The Sri Yantra. Easily to copy, and on internet being public domain.
By printing it and using as art in your home it gives the radiation of the hindu gods.

A citation of astroved.com:
Sri Chakra Yantra or the Sri Yantra is your magical wand to invoke all the riches.
Let your life be the synonym of prosperity!
Goddess Tripura Sundari adds enchantment, prosperity and happiness to your life.
The energized radiations from the Yantra
stimulate your thirst to acquire money, material comforts and wealth.
It lays a luck strewn path for your success!