Which denominations believe in modern (ongoing) revelation?

The Baha’i Faith believes in progressive, on-going revelation.

Progressive revelation is a core teaching in the Bahá'í Faith that suggests that religious truth is revealed by God progressively and cyclically over time through a series of divine Messengers, and that the teachings are tailored to suit the needs of the time and place of their appearance.[1][2] Thus, the Bahá'í teachings recognize the divine origin of several world religions as different stages in the history of one religion, while believing that the revelation of Bahá'u'lláh is the most recent (though not the last—that there will never be a last), and therefore the most relevant to modern society[1].

Bahá'ís believe God to be generally regular and periodic in revealing His will to mankind through messengers/prophets, which are named Manifestations of God. Each messenger in turn establishes a covenant and founds a religion. This process of revelation, according to the Bahá'í writings, is also never ceasing,[1]

Bahá'u'lláh, the founder of the Bahá'í Faith, explained that the appearance of successive messengers was like the annual coming of Spring, which brings new life to the world which has come to neglect the teachings of the previous messenger.[1]

[1] Smith, Peter (2000). "Progressive revelation". A concise encyclopedia of the Bahá'í Faith. Oxford: Oneworld Publications. pp. 276–277. ISBN 1-85168-184-1.

[2] Effendi, Shoghi (1974). Bahá'í Administration. Wilmette, Illinois, USA: Bahá'í Publishing Trust. p. 185. ISBN 0-87743-166-3.

The concept of progressive revelation is further explained in the Bahá'í writings in numerous metaphors. These metaphors include the daily and seasonal cycle and the progression through a school. Source: https://en.m.wikipedia.org/wiki/Progressive_revelation_(Bah%C3%A1'%C3%AD)

Here is another consideration, although I don’t know whether a revelation dating to the 18th century counts as “modern.” The gentleman who wrote the theology that became the basis for the Swedenborgian movement claimed to have received a new revelation in the 1700’s:

Emanuel Swedenborg (1688-1772) claimed to have received a new revelation from Jesus Christ through continuous heavenly visions which he experienced over a period of at least twenty-five years. In his writings, he predicted that God would replace the traditional Christian Church, establishing a 'New Church', which would worship God in one person: Jesus Christ... The movement was founded on the belief that God explained the spiritual meaning of the Scriptures to Swedenborg as a means of revealing the truth of the Second Coming of Jesus Christ. Swedenborg claimed divine inspiration for his writings and followers believe that Swedenborg witnessed the Last Judgment in the spiritual world, along with the inauguration of the New Church. Source: https://en.wikipedia.org/wiki/The_New_Church

Neither do I know if ongoing revelation is a feature within this movement. However, the above would qualify as a revelation. Perhaps Lee Woofenden would care to comment?

The Society of Friends (Quakers) generally believe that each person has what they term the Divine Light, or "that of God," within them. Because God is within and directly accessible to everyone, Friends generally value direct experience of the divine over religious texts (particularly the Bible).

The Dukhobors (a Christian movement that originated in Russia and Ukraine probably sometime in the 18th century and many of whom emigrated to Canada around the turn of the 20th century) have what is in many respects a similar belief pertaining to the value of religious texts, though they have historically tended to emphasize charismatic leaders and institutions over direct individual revelation (I should emphasize that this is a tendency, not a universally-true statement).