I'm looking for something that is cheap, does a good job, and doesn't require me to buy 50 plug-ins to get the one I need. I'm hoping to find one that interpolates frames for slow motion so it really looks smooth.

Any ideas, please let me know.

Thanks

People report good results with Twixtor:

http://www.revisionfx.com/rstwixtor.htm

Read the reviews on their site however, it sounds like it could be difficult to understand and use, depending on what you want to do. I have never used it myself so I don't really have an opinion one way or the other.

Have you tried FCP's speed change capability. I find it generally suits my own needs. Another thing to try when using FCP's built-in feature is to also go to the motion tab in the viewer window and click on the frame blending checkbox. It helps to smooth out the jerkiness that you get when slowing footage way down. You sacrifice a little sharpness, but personally I think it looks more pleasing.