JOEL - Like you said, one manifestation of the gift of tongues(and most important reason for it) is for the purpose of preaching among those whose language is not understood. Joseph Smith said:

"When devout men from every nation shall assemble to hear the things of God, let the Elders preach to them in their own mother tongue, whether it is German, French, Spanish or Irish, or any other, and let those interpret who understand the language spoken, in their own mother tongue, and this is what the Apostle meant in First Corinthians 14:27." (December 26, 1841. TPJS 195)

A second manifestation of tongues is when someone speaks in the pure Adamic language. We are told in modern revelation that our first parents were taught by God to read and write in a language that was "pure and undefiled" (Moses 6:6). The Adamic language continued in use until the time of the Tower of Babel, at which time the speech of the people (except for the Jaredites, who took this sacred tongue with them to America) was confounded (Genesis 11:1-9; Ether 1:33-37; Smith, Way to Perfection, 67-69).
The Adamic tongue was a powerful language that communicated not only words and ideas but also the power of God.

"And so great was the faith of Enoch that he led the people of God, and their enemies came to battle against them; and he spake the word of the Lord, and the earth trembled, and the mountains fled, even according to his command; and the rivers of water were turned out of their course; and the roar of the lions was heard out of the wilderness; and all nations feared greatly, so powerful was the word of Enoch, and so great was the power of the language which God had given him" (Moses 7:13).

One of the first demonstrations of the use of the Adamic tongue in our dispensation took place early in the history of the Church. Brigham Young relates the following story:

"In September, 1832, Brother Heber C. Kimball took his horse and wagon, Brother Joseph Young and myself accompanying him, and started for Kirtland to see the Prophet Joseph. We visited many friends on the way, and some Branches of the Church. We exhorted them and prayed with them, and I spoke in tongues. Some pronounced it genuine and from the Lord, and others pronounced it of the devil.
"In the evening a few of the brethren came in, and we conversed together upon the things of the kingdom. He called upon me to pray; in my prayer I spoke in tongues. As soon as we arose from our knees the brethren flocked around him, and asked his opinion concerning the gift of tongues that was upon me. He told them it was the pure Adamic language. Some said to him they expected he would condemn the gift Brother Brigham had, but he said, 'No, it is of God, and the time will come when Brother Brigham Young will preside over this Church" (Millennial Star 25 [3 January 1863]: 439).

"During the millennium, it appears that men will again have power to speak and write the Adamic language. Of that day the Lord says he will "turn to the people a pure language, that they may all call upon the name of the Lord, to serve him with one consent." (Zeph. 3:9.) In some instances when the saints speak in tongues, the language impressed upon them by the power of the Spirit is the pure Adamic tongue." (Mormon Doctrine, Bruce R. McConkie p 19)

The gift of tongues is not something for the entertainment of members, nor is it for the purpose of creating awe or to increase faith in those who are weak. The gift of tongues and the interpretation of tongues are given for the purpose of helping to build up and strengthen the kingdom of God. (Answers to Gospel questions, vol 2 p 26)
(See also "Alive in Christ: The Miracle of Spiritual Rebirth", Robert L. Millet)

BLIZZ - I try to obey the speed limits and I try not to ever lie or break any laws whatsoever, but the truth is...it seems to be virtually impossible to obey every law to the letter. Where does God draw the line and where should we draw the line without beating ourselves up about every little thing? I want the Lord to be proud of me, and I want to make sure I am Celestial worthy. How can I know when I need to obey to the letter of the law, or when its ok to "let it slide"?
Thank you so much and again, any references are greatly appreciated.

JOEL - We can't be completely perfect in this life. As far as we are concerned, on our path to perfection in this life there is no solid permanent line that God has drawn; that line exists in the eternities. The trick is to learn the doctrines and commandments that God has given us in this life and then draw our own line and keep moving it ever forward as we perfect ourselves in individual things and grow in righteousness.
This is a very personal thing between you and God. As long as you keep moving that line forward you are on the right path and God will be proud of you for doing this and you can be pleased with yourself, hopefully without losing humility. The Holy Ghost is very important in helping you determine when to live the letter of the law or the spirit of the law. Believe it or not I'll bet President Hinckley worries about this himself sometimes. Of course his line is a lot further down the road than ours :-)
I think the best advise on this subject is in a talk called "Perfection Pending" by Elder Russell M. Nelson.
(Russell M. Nelson, “Perfection Pending,” Ensign, Nov. 1995, 86)