* THE SPRINGHILL SINGING DISASTER (Playwrights Horizons): A solo piece written and performed by Karen Trott, in which she charmingly relates her misadventures in show business. Describing her disastrous beginnings as a folk singer and her later career as an actress (she had roles in such Broadway productions as ''Strider'' and ''Barnum''), Trott has taken situations that would have permanently discouraged a lesser human and found the humor in them. Displaying both comic and musical prowess and doing near-perfect imitations of Joni Mitchell and Judy Collins, she may just find with this show the success that has so far eluded her.
* ECSTASY (Judith Anderson Theatre): A 1979 play by British filmmaker Mike Leigh (''Naked,'' ''Life Is Sweet,'' ''High Hopes'') now receiving its American premiere, thanks to a spunky new theater company called The New Group. The production, which won an Obie last season, has just reopened Off Broadway for a commercial run. It concerns a group of working-class Brits hanging out one night in a run-down, suburban-London flat; like most Leigh works, it is more concerned with texture than plot. But the dialogue is sometimes hilarious, the characterizations are wonderfully precise, and it is expertly acted, particularly by Carolyn Seymour in the central role. This is a vivid production of a play you won't soon forget.
* LONELY PLANET (Circle Repertory Theatre): Steven Dietz's allegorical, sometimes absurdist play about AIDS presents two friends, Carl and Jody. They meet periodically in Jody's store, which specializes in selling maps. The first half is largely concerned with Carl bringing endless numbers of chairs into the store; but, in a work such as this one, a chair is not just a chair. The play is frustratingly indirect and whimsical and is too slight to sustain its two-hour running time. But there are moments and lines of dialogue that resonate deeply, and Denis O'Hare and Mark Shannon deliver deeply felt performances.