there is no inexpensive way to do this on EITHER PC or MAC. Any software solution (other than the built in scopes that come with FCP or AVID, for example - both which are terrible) requires a capture card, and this adds up to an expensive product.
For example, on a PC, you can use the Hamlet VidScope software (about $995), but unless you are using DV as a signal, you need a CAPTURE CARD to be able to see a SD or HD signal - this means that you need a dedicated PC, as well as a dedicated capture card (like a Decklink card) to have a fully functional waveform/vector display. If you are on a MAC, you can use Divergent Media Scope Box, but this too requires a dedicated MAC, along with a dedicated capture card (like a Decklink) for this to work.
By the time you get done, you will spend about $4000 to make this work (either MAC or PC). For $5000 you can get the new Tektronix WVR-5000, which is a fully functioning waveform/vectorscope.
So, now you say "this can't be - there MUST be a cheaper way to do this" - well, there is - it's called EBAY. But for a NEW solution, that will accurately monitor SD and/or HD signals, there is no "$500 plug in" that will monitor your critical video levels in real time. Solutions that you hear about, like OmniTek, are wonderful and EXPENSIVE.
"So, why doesn't someone make something that works that is not expensive" - WHAT A GREAT IDEA !!!! This needs to be the "next" product from a company like Blackmagic, AJA, or Convergent Designs (or one of those guys that make cool inexpensive little boxes).