There are several words which are used both as adjectives and as adverbs.
Some adjectives formed by adding –ly to nouns are also used as adverbs.

Examples are: early, daily, weekly, yearly, hourly, quarterly, half-yearly etc.

This is a weekly journal. (Here the adjective weekly modifies the noun journal.)
It comes out weekly. (Here the adverb weekly modifies the verb comes.)
We boarded an early train. (Here the adjective early modifies the noun train.)
I get up early. (Here the adverb early modifies the verb get up.)

Adjectives ending in -ly

Note that the following words can be used only as adjectives and not as adverbs: beastly, costly, cowardly, deadly, friendly, likely, brotherly, kingly, leisurely, lovely, lively, womanly, princely, scholarly, silly, ugly and unlikely.

She offered some friendly advice. (Here the adjective friendly modifies the noun advice.)

There are no adverbs friendly/friendlily.   Instead we use an adverb phrase.

She offered some advice in a friendly manner.

Words used both as Adjectives and Adverbs

The following words may be used both as adjectives and adverbs. Some examples are given below; for more detailed information, check in a good dictionary.

Fast

Modern man leads a fast life. (Here the adjective fast modifies the noun life.)
He drove the car fast. (Here the adverb fast modifies the verb drove.)

Half

Each of us has half ownership in the property. (Here the adjective half modifies the noun ownership.)
She was half crying, half laughing. (Here the adverb half modifies the verbs crying and laughing.)
She is half Spanish, half French. (Here the adverb half modifies the adjectives Spanish and French.)

Straight

Don’t expect to get a straight answer from her. (Here the adjective straight modifies the noun answer.)
She went straight home. (Here the adverb straight modifies the verb went.)

Just

The train just left. (Adverb)
He arrived just in time. (Adverb)
God is just. (Adjective)

Late

The meeting is scheduled for late evening. (Adjective)
He arrived late. (Adverb)
She worked late. (Adverb)

Low

We are low on supplies. (Adjective)
Her voice was low. (Adjective)
She turned the gas down low. (Adverb)
The helicopter flew low over the trees. (Adverb)

Most

This is the most expensive gadget I have bought. (Adverb)
Most people enjoy playing with kids. (Adjective)

Clear

They moved clear across the country. (Adverb)
She has a clear complexion. (Adjective)

Clean

He is very clean in his habits. (Adjective)
We want to play the game clean. (Adverb)