This is a work in progress, based on my own experiences trying to wrangle tickets.
San Francisco Opera: There are 200 standing room tickets for each performance, and 150 of these go on sale at 10 am the day of the performance. The remaining 50 are sold 2 hours before the performance. Tickets are $10, cash only, and each person may buy 2 tickets. They let standees in on the south side of the building, across the street from the Symphony on Grove Street, 70 minutes before the curtain time. One enters by number, and there is a numbered line painted on the ground outside. The standing room areas are on the orchestra level and the back of the balcony. Student, senior, and military rush tickets may also be available.
Seattle Opera: There are standing room tickets available on the day of a given performance, beginning at 9am on Wednesdays and Fridays, and at noon for Saturdays and Sundays. Tickets are sold in person at 1020 John Street or over the phone at (206) 389-7676, and are also available at the opera house an hour before the show. Tickets are $12, and standing places are numbered, and are located in the back of the orchestra level, only on the sides. Titles may be obstructed, so one might want to read the libretto beforehand.
Los Angeles Opera: No standing room, rush available.
San Diego Opera: No standing room.
Lyric Opera of Chicago: No standing room. Student rush tickets may be available through Lyric Opera NExT. For sold-out shows, donated tickets may be available the day of the performance and are sold after 2pm (11am for matinées) in-person at the box office or on the phone.
The Met: Orchestra standing room tickets are available the day of a given performance, and can be bought in person or via the Internet or phone at 10am. The standing places are numbered. During the week, one should wait on the Plaza Level, near the box office doors. On Saturdays, one can take a number from Josephine on the Concourse Level.
Royal Opera, London: Standing room tickets are sold along with the regular tickets, and are located in the stalls circle, balcony (restricted view), lower slips (retricted view), and amphitheater. Tickets range from 6-11 pound. 67 day seats are sold in person on the day of each performance at 10am, and only one is sold per person. The line forms under the covered arcade in the corner of Covent Garden Piazza. Tickets range from 6-50 pound. Half-price tickets are offered for unsold tickets four hours before a performance. Low-price standby tickets may also be available.
Bayerische Staatsoper (Bavarian State Opera in Munich, Germany): Standing room (Stehplätze) tickets are sold along with the regular tickets, three months before a given performance. Tickets range from 4-21 Euro. Partiturplätze and Hörerplätze, special seating where one can read the score or just listen, are also sold with the regular tickets, and range from 4-16 Euro.