Just as 死了 can be used to intensify negative adjectives, 极了 (jíle) can be used to intensify positive adjectives.This is used in spoken, colloquial Chinese. 极 means "extreme" or "utmost", so using this grammar pattern takes your positive adjectives to the extreme!

Structure
It's as simple as using 极了 after the positive adjective.

subject+positive Adjective+极了

Note that this structure is a degree complement (called 程度补语 in Chinese)， and one of the few that you can use after the adjective and without adding a 得.

Examples

Remember to add the 了, since this is sort of an exclamatory statement similar to 太好了. It would be incorrect without it.
× 今天 我 的 精神 好 极。
√ 今天 我 的 精神 好 极了。