Ничего страшного (Nichevo strashnava). Literally it means "nothing scary", but it's closer in the meaning to "no catastrophy, so basicly means "no problem". You can use it when someone is apologising to you.

Нет проблем!

Yes, we say Nyet problEm!

Нет проблем (nyet problem) - direct translate. Не проблема (ne problema) - other translate. Не вопрос (ne vopros) - mean "not big deal"

or без проблем =)