Formal:
* It is thought
* It is generally accepted
* There is a consensus that
* It is widely held that
* The prevailing view is
* It is commonly understood that
* It is understood that
* It is considered that
* It is reported that
* It is stated that
Informal:
* People think
* Many people believe
* Some people say
* The rumor is
* Word on the street is
* It's said that
* Apparently,
* Supposedly
More specific:
* According to [source],
* Research suggests that
* Studies have shown that
* Experts agree that
* Historical records indicate that
The best alternative will depend on the context and tone of your writing.