open mike 
(also open mic)
noun [often as modifier]
A session in a club where anyone is welcome to sing or perform stand-up comedy: if this was an open mike session she would be booed off stage...
- Any performers are welcome to share their skills at the open mike session - musicians, dancers, poets, puppeteers, raconteurs and actors.
- The event will also include an open mike session and any Aboriginal performers are welcome to turn up and share their talents.
- What we got was open mike night at the TSN Comedy Club.