Not available on Saturday evenings or Sunday matinees or opening nights. Not available for some shows. The theater listings of the Los Angeles Times Calendar Section and the L.A.Weekly both contain extensive listings from the smaller theaters. The prices are reasonable, and discounts are usually available for students. Everything from jazz, classical, dance and Mariachi.
Tickets can be really cheap.
Bring a picnic. The Greek Theater in Griffith Park hosts a variety of performances from Shakesphere to Modern Rock. Visit their home page for current season and ticket info. No big productions, but it's close and the shows are usually pretty good.