Gemara

Question: How did Delilah know that Shimon "had told her all his heart"?

Answer #1 (R. Chanin): Truth is recognizable.

Answer #2 (Abaye): Shimshon said "I am a Nazir Elokim." She knew that a Tzadik would not say Hash-m's name in vain.

Berachos 33a (Rav): Anyone who says an unnecessary blessing transgresses "Lo Sisa (Es Shem Hash-m Elokecha la'Shav)."

Nedarim 7b (Rav): If David hears Reuven say Hash-m's Name in vain, he must excommunicate Reuven. If he does not, he should be excommunicated.

Temurah 3b: "Im Lo Sishmor...; V'Hifla...." teaches that one who curses someone with His name is lashed.

Question: Perhaps it teaches about one who says His name in vain!

Answer: Cursing is also a vain mention of His name (in any case he is lashed)!

Clarification: We meant, perhaps lashes suffice for saying His name in vain, but not for cursing with His name, for this additionally pains people!

Answer: This cannot be. It says "Lo Sekalel Cheresh." Granted, if "v'Hifla..." is the punishment for cursing, then "Lo Sekalel Cheresh" is the Azharah (warning);

But if "v'Hifla..." is for saying His name in vain, then what is the warning?

Suggestion: Perhaps "Es Hash-m Elokecha Tira" is the warning!

Rejection: That is an Aseh. An Azharah must be a Lav.

Rishonim

Rambam (Hilchos Berachos 1:15): If one blesses unnecessarily, he says Shem Shamayim l'Vatalah. He is like one who swore in vain. One may not answer Amen after him.

Magen Avraham (OC 215:6): One who says an unnecessary blessing transgresses "Lo Sisa" (Berachos 33a). Tosfos (Rosh Hashanah 33a DH Ha) and the Rosh (Kidushin 1:49) hold that this is mid'Rabanan. The verse is merely an Asmachta. The Rambam holds that it is mid'Oraisa. Temurah 4a supports him. Tosfos says that there it discusses Shem Shamayim l'Vatalah not in a Berachah.

Kesef Mishneh (Hilchos Milah 3:6): Chachmei Lunil asked the Rambam why we do not bless on Milas Androginus, which is a Safek mid'Oraisa. He answered that Chachamim enacted the Berachah, and they said that one who blesses needlessly transgresses Lo Sisa.

Question (Kol Eliyahu 10, cited by R. Akiva Eiger): The Rambam holds that the Torah is lenient about a Safek mid'Oraisa. If one is unsure whether or not he blessed Birkas ha'Mazon, he is exempt. If the Torah forbids unnecessary Berachos, why did Chachamim obligate him to bless? Why did Chachamim enact Berachos, if this is an Isur Torah?

Answer (Kol Eliyahu): The Rambam holds that the Torah is lenient about a Safek when there is no Chazakah. If there is a Chazakah (he was obligated to bless), we follow it. However, some say that this applies only to a Safek for which one brings Asham Taluy. We must say that the Torah authorized Chachamim to decide when one may bless, like we say about Melachah on Chol ha'Mo'ed. In a Teshuvah (105) the Rambam explicitly forbids unnecessary Berachos mid'Oraisa, contradicting what he wrote in the Teshuvah (26) cited by the Kesef Mishneh.

Eshel Avraham (cited in Otzar Meforshim Friedman Shulchan Aruch 215:1): The Rambam agrees that it is only mid'Rabanan, unlike the Magen Avraham.

Rambam (4:10) If one blesses on a food and it fell and was burned or taken away by a river, he says Baruch Shem... in order that his Berachah will not have been Shem Shamayim l'Vatalah.

Tosfos (Rosh Hashanah 33a DH Ha): The Gemara (Temurah 3b) expounded that Shem Shamayim l'Vatalah is forbidden (mid'Oraisa). This does not apply when it was in an (invalid) Berachah.

Rosh (Berachos 6:20): If one said a Berachah l'Vatalah, he says Baruch Shem Kavod.... If he only said Baruch Atah Hash-m, he finishes Lamdeni Chukecha, to avoid a Berachah l'Vatalah.

Question: How can one excommunicate one who said Shem Shamayim l'Vatalah? Perhaps he said Baruch Shem..., and fixed his transgression!

Answer (Einayim l'Mishpat, ibid.): Perhaps Baruch Shem... does not fix the transgression. It is merely Teshuvah. Tosfos (Berachos 39a DH Botzar) holds that it helps only for a Berachah l'Vatalah (which is not mid'Oraisa). The Rambam holds that it helps only if he was Shogeg.

Mabit (1:262): A Shevu'ah not to wear a turban is not considered Shem Shamayim l'Vatalah. The Ramban explains that Shevu'as Bituy is a secular matter; he includes it in the Mitzvah "uvi'Shmo Tishave'a". The Rambam obligates one who mentioned Shem Shamayim in a Shevu'as Havai, in which he is exempt (from observing anything), but not in Shevu'as Bituy, which he must fulfill. In Hilchos Shevu'os he obligates for Shevu'as Shav or Sheker, but not for Bituy. The Medrash permits swearing with His name only if one fears, serves and clings to Hash-m. I.e., then it is a Mitzvah, but it is not Shem Shamayim l'Vatalah if one who lacks these Midos swears.

Rambam (Hilchos Shevu'os 12:9): If Shimon said Hash-m's name needlessly, or swore falsely, or made an unnecessary Berachah, he transgressed saying Hash-m's name in vain.

Rambam (11): One may not mention Hash-m's name needlessly even without a Shevu'ah. It says "l'Yirah Es ha'Shem ha'Nichbad veha'Nora". Fearing His name includes not saying it l'Vatalah. Therefore, if one said His name l'Vatalah, he should rush to praise and glorify His name, so the mention will not have been l'Vatalah. He should say Baruch Shem... or 'Gadol Hu u'Mehulal Me'od' or something similar.

Rosh (Kidushin 1:49): R. Tam says that women may bless on a Mitzvas Aseh sheha'Zeman Gerama. Even though they are exempt even mid'Rabanan, since they intend for a Berachah, it is not Lo Sisa.

Poskim

Shulchan Aruch (OC 156:1): One must be careful not to say Hash-m's name needlessly, for where this is common, death is common.

Shulchan Aruch (206:6): If one blesses on a fruit and fell and became ruined, one says Baruch Shem... for having said Shem Shamayim l'Vatalah. If he had said only Baruch Atah Hash-m, he concludes "Lamdeni Chukecha", which is like saying a verse.

Shulchan Aruch (215:4): If one blesses unnecessarily, he says Shem Shamayim l'Vatalah. He is like one who swore in vain. One may not answer Amen after him.

Mishnah Berurah (19): This is even if he said the Berachah correctly, just he did not need to say it, and all the more so for Shem Shamayim l'Vatalah not in a Berachah. He is liable for names other than His four-letter name. One may say His name in other languages only to give praise or thanks.

Mishnah Berurah (20): Most say that Berachah l'Vatalah is only mid'Rabanan. All agree that Shem Shamayim l'Vatalah without a Berachah transgresses "Es Hash-m Elokecha Tira".