To begin with - Начнём с того, что
Frankly speaking - Откровенно говоря
To be more precise - Чтобы быть более точным
To be quite frank - Совершенно откровенно
To make matters (still) worse - Что ещё хуже
It goes without saying - Само собой разумеется
To cut a long story short - Короче говоря
To put it in a nutshell - Вкратце, в двух словах
To say the least - Без преувеличения
So to speak - Так сказать
To put it mildly - Мягко говоря
Strange as it may seem - Странно как это не кажется
To sum it up - Подводя итоги
To tell the truth - По правде говоря
As far as I know - На сколько я знаю
Strictly speaking - Строго говоря
By the way - Между прочим

Пример употребления вводных оборотов в речи:

A: By the way, what do you think of Bob Lanski?
B: To begin with, I don't know him well enough. To tell the truth, he seems to be clever and hardworking. And what is your opinion of him?
A: Strictly speaking, Bob is not as clever as he looks. To put it mildly, he is a bit lazy and absent-minded, so to speak. Strange as it may seem, a lot of people like him.
B: Yes, as far as I know he is quite popular, especially with women.