I think you're going to need to be a little bit more specific about exactly what you want to do. There are a half dozen good answers to your original post, but each one is a good answer to a slightly different problem 🙂
If it's live playback of video sources for a production, or if you're videotaping a live production with minimal edits and all you want is to fade in/out, consider a video mixer, as some people have said. There are lots of good modern units, but even some older ones will do the job nicely. I had a Panasonic WJ-AVE5 mixer for a while which I picked up on eBay for about $200 USD. As far as effects and wipes went, it was kind of cheezy, but if all you needed to do was fade in/out from black, or do A/B rolls, cut to/from sources, etc., it did a fine job. It had S-Video inputs and outputs. You can get mixers with DV in/out nowadays, but they cost a bunch more.
Fun toys, in any case 🙂