27" iMac as old as the late 2009 will support TDM (Target Display Mode), and TDM is available on Snow Leopard, as long as the iMac supports TDM. If your iMac is 27" model, it is more likely to support TDM.
If it doesn't support TDM, there is a DIY method, which requires a lot of hardware modification, including soldering and acquiring appropriate LCD controller so that its built-in monitor can be used as a normal external monitor. The work involved, the time required, and the cost of parts required for the procedure are often too prohibitive and not worth the effort. For all that, it's probably better to buy a brand new monitor for less. I've once tried to convert the beloved iMac G4 for such use, but ultimately failed, although few people have tried and succeeded through extreme measures. See The 17" iMac G4 to DVI - The Easiest Method Ever (So Far).
Finally, if the iMac is still running, I recommend the software solution. It allows the old iMac to be used as a secondary display to an existing PC or Mac (through VNC and the like). I've had success with Screen Recycler , which cost around $30, but is well-worth the price. If it is an older PPC-based iMac and it runs Tiger, I recommend ScreenSpanningDoctor. Newer software, such as AirDisplay works as well, but it requires Mountain Lion.