(Disclaimer: This answer is from a Protestant perspective and may not fully address the Catholic perspective.)
Huh. It seems like most of the answers are missing an important facet of the questions being asked.
Do Catholics & Protestants believe that God lets morally good people from other religions go to heaven? Do good people practicing Islam and Hinduism go to heaven?
Of course He does! All zero of them are in Heaven!
Your question starts with a flawed premise, which is that there are "good" people (Christian, or otherwise). There are none.
All have sinned and fall short of the glory of God (Romans 3:23)
There is none who does good, not even one (Psalm 14:3)
All, both Jews and Greeks, are under sin [...] None is righteous, no, not one (Romans 3:9-10)
If we say we have no sin, we deceive ourselves, and the truth is not in us (1 John 1:8)
If you, O Lord, should mark iniquities, O Lord, who could stand? (Psalm 130:3)
The purpose (well, one purpose, but I won't go into the whole curb/mirror/guide lesson) of the Law is to show that everyone is sinful. God is Just in condemning everyone to eternal punishment, even Christians. Even Abraham, Moses, and David.
For further reading, see also Wikipedia's article (and any other) on Original Sin.
In His mercy, however, God sent His Son to suffer the punishment for sin in our place, so that, by faith, we might be forgiven and spared the punishment we justly deserve. Christianity is about knowing and accepting this Grace, and it is the only way one can be saved.
For by grace you have been saved through faith [...] not a result of works (Ephesians 2:8-9)
We are saved not because we are "good" (we aren't!), but by faith, and this is seen even in the OT. Abraham was not righteous in and of himself; rather, "Abraham believed God, and it was counted to him as righteousness" (Romans 4:3). Those that trust in God are saved, by faith, not by being "good".
How, then, does a Bhuddist know or trust in God? How would they come to faith in God's promise through Christ?
[Jesus said] "I am the way, and the truth, and the life. No one comes to the Father except through me." (John 14:6)
For God so loved the world, that He gave His only Son, that whoever believes in Him should not perish but have eternal life. (John 3:16)
Everyone who believes in [Jesus Christ] receives forgiveness of sins through His name (Acts 10:43)
If there were truly a human without sin, that person would indeed have no need of Christ's atonement. (I'd point out that such a person would necessarily be Jewish anyway, because belief in any other religion would be a violation of the First Commandment and therefore sinful. And they'd have to perfectly keep the entire Jewish law.) Aside from Christ, however, no such person exists, has ever existed, or will ever exist.
We are not saved because we are "good". No one is "good". We are saved because we trust in God (and Christ). If your faith is in Hinduism, or in "Allah", or in whatever else, you are putting your trust in something besides Christ, which has no power to save, and God will (according to non-Universalists, anyway) justly judge you accordingly.
Naturally, most religions claim the power of salvation. Here's a fun experiment, however: how many teach that you, by your own power, are utterly helpless and that it is only by the intercession of someone else that you can be saved? If the answer is "one" (Christianity), might that uniqueness have some deeper meaning, perhaps attesting to the veracity of that unique religion over all others?
I would highly commend you to read through the NT, starting, perhaps, with Romans, and also to have a look around at some other articles on Sola Gratia. Understanding the underlying flaw in your question will hopefully help you to also understand its answer.