Edit:
I have found a possible alternative to the gamma correction device listed in the 2nd post. Toshiba makes the TA8696F gamma correction IC for LCD TVs. I was thinking we could use it in a post-component source device.

Here is an application circuit from the datasheet found HERE

Alternative AVS Forum device:

If you want gamma correction enhancement of your 480P, 720P, 1080i signals the guys over at avsforums have developed a stand alone box just for that purpose. It is to be used with a transcoder in line between your HD source and the transcoder. No HTPC needed.

However, the gamma correction is not adjustable. It is set to a predetermined optimal level to bring out the details in shadows while keeping blacks black.

You can either build your own, or a guy on the forums is building them for people for $200.

AVS FORUMS GAMMA DEVICE THREAD

Edit: fixed the link

Link should be

I found another simple gamma correction circuit on avsforums : Thread here
Read the thread to learn how to swap values for different gamma curves.

Three of these circuits are needed...one each for R, G, B.

Click to view attachment