I have a 600EX-RT Canon flash, and am looking for a cheap Yongnuo flash to be used as a slave to the Canon. Do you know any Yongnuo than can reliably be used as a slave to my 600EXRT ?
Depends on what method of slaving we're talking about and your definition of reliable.
If you mean as a dumb optical slave, where the main flash burst from the 600EX-RT can trigger the flash, then all of the YN flashes have "dumb" slave modes on them.
If you mean as a wireless eTTL near-infrared optical slave, then any of the Yongnuo flashes with EX in the name can be used as a slave with the 600EX-RT as an optical master.
If you mean as an RF slave in the Canon RT system, then the YN600EX-RT is a cheap clone of the Canon 600EX-RT, and the Yongnuo YNE3-RX radio receiver can be added to Canon and Yongnuo (and one assumes anybody else's) eTTL and menu-commandable models (e.g., a YN-568EXII or YN-500EX) for triggering and control from an RT master unit.
A note: with flashes you can have any of the following two things: lots of features, a low price, or rock solid reliability. You want cheap and rock solid reliable, then an RT feature set via a cheap Chinese clone may not be the right path. There are reports on early 600EX-RT units being great and reliable. There are also reports of them having dead scanlines on the LCD screens, battery drain issues, and making weird noises. If you have to have something rock-solid reliable, then a Canon 600EX-RT is probably your best bet. If you just need hobbyist usage sufficient, then Yongnuo's a no-brainer.